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Ufte mn i 

Scanned from the collection of 
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Jeff Joseph 




The Blue Book of Projection 


In Two Volumes 


Printed By 

C. J. O'BRIEtf Inc. 

Union Printer 




Copyright 1927 

Chalmers Publishing Company 

New York. 

All Rights Reserved. 


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Fifth Edition 



First Printing, March, 1927 
Second Printing, January, 1928 



LIKE the four editions which have gone before, this, the 
Fifth Edition has been compiled with the single idea up- 
permost of providing competent information upon, and 
instruction in motion picture projection and the various things 
directly allied thereto. 

The reason for making this edition in two vohimns is that we 
may be able to keep the work as up to date as possible with 
a minimum of cost to its purchasers. We have therefore so 
far as practicable placed in Volume I such matter as seldom 
or never changes. In Volumn II we have placed matter cov- 
ering projection equipment, which is subject to more or less 
continual change. 

By this plan we believe the theatre manager and projection- 
ist may have a complete text book on motion picture projec- 
tion, acceptably up to date, by the occasional purchase of a 
second volume which will" be revised from time to time as 
equipment changes. 

In Volumn I of this Fifth Edition you will find much matter 
which is identical with matter in the fourth and third editions 
and even the second of the Handbooks. This is for the reason 
that it covers matters which are fundamental and do not 
change. The fundamental work having been accurately and 
painstakingly done in the first place, can not be improved by 

As in previous editions there is in various parts of this edi- 
tion a small amount of what might appear to be repetition. 
This is because it often seems advisable or necessary to treat 
of certain things in several different connections. It appears 
only where the author believes it essential to the best interests 
of the work as a whole. 


Volume I 

IMPORTANT NOTICE: We have, after much deliberation, 
adopted a new, and so far as we know, a unique method of 
indexing this work. Instead of merely indicating where a sub- 
ject may be found, as has heretofore been done, we have 
indicated the main subject by a prominent heading, and 
then, have asked questions concerning various details of that 
subject, and indicated the page upon which the answer will 
be found. 

Also by using bold face side headings we have shown the 
more important divisions of the main subject. Altogether we 
believe this plan will prove of very great value. 

IMPORTANT: The main subjects have been placed alpha- 
TERMINATION as to the location in the question index, 
thus: Electric Motors will appear as "MOTORS, ELECTRIC" 
The screen will appear as "SCREEN, THE," etc. 

get the general plan of it fixed in your mind, after which 
we believe you will be able to quickly find any desired topic, 
or any detail of any topic. 

Aberration, Chromatic 

(See Light Action) 


(See Projection Room) 

Angle, Projection ,... 294 

Arc Controllers 

(Also see pages 608 to 624, Vol. II.) 
Is the use of arc controllers to be commended 372 and 608 
Will a hand- fed arc lamp give us steady screen illu- 
mination as the lamp fed by a good arc controller?.. 372 
Upon what principle of the electric arc does arc con- 
trollers depend for their action? 372 


Arc Light Source, The Electric 

(Also see page 784, Vol. II.) 

In a projection arc circuit, name the various items 
offering resistance to current flow 54, 57, 393 

When current is forced across from one electrode of an 
arc to the other, something is consumed. What is 
it? 56 

Will all projection arcs operate best at the same vol- 
tage? 56, 400 

How would you proceed to calculate the resistance of 
an arc ? . , 57 

What is the formula for calculating projection arc 
voltage ? 57 

Rule o* Thumb 

What is the "Rule o' Thumb" for calculating voltage, 
amperage or resistance of a projection arc? . . 59 

What extremely important factor enters into all prob- 
lems connected with electrical action, and especially 
as concerns the production of light by. electricity ?... . 60 

Uneven Luminosity 

Does the floor of the crater of an electric arc present 

a surface of even luminosity? 171 

What is the reason the floor of the electric arc has not 

even luminosity ? 171 

Why is the image of the light source upside down at 

the spot? 171 

Does a crater form on only one carbon with D. C. and 

on both with A. C? 392 

AC and DC Arc 

Will an A C arc deliver fifty per cent, as much 
light to the screen as a D C arc of equal am- 
perage? .....381, 392 

When A. C. is used at the arc should only the upper 
crater be used ? » . . . 392 

Name two points of difference as between D. C. 
and A. C. proj ection light 393 

Are the violet lines in the arc stream said to be 
brighter than the crater floor itself ? 393 

Does the flow of current through the gas stream 
cause it to have luminosity regardless of its tem- 
perature ? 3^3 



Cause of Light 

To what is the light producing power of the crater 
floor entirely due ? 393 

Why does the crater form only on the positive car- 
bon? 393 

What resistance do the various elements of the 
electric arc offer? 393 

Crater Temperature 

Upon what factors does the total light giving power 
of an electric arc depend? 394 

How may the total light giving power of a crater 
floor be computed? 394 

When cored carbons are used, what is the approxi- 
mate candle power of a crater floor per square 
millimeter? 394 

May the square millimeter light giving power of a 
crater floor be decreased appreciably by using a 
too-large carbon ? 394 

Provided the right diameter carbon for the amper- 
age be used, does change in amperage alter square 
millimeter brilliancy of crater floor ? 394 

Different Carbons Vary- 
Given a 55 degree crater angle, does crater increase 
per added ampere vary with different makes of 

carbon ? 395 

In what way may you measure crater diameters ac- 
curately? 395 

What crater area does different amperage produce 

on National carbons ? 395 

Does crater area increase proportionately with am- 
perage increase? 396 

In what proportion does the light giving power of 
a crater increase with doubling of amperage, cor- 
rect size carbons being used in both cases? 396 

Do observed results as to crater area, and results 
calculated as per formula on page 396 agree ? 396 

High Amperage Wasteful 

When using the ordinary straight arc and a correct 
lens system for projection, is there loss of light 
below 60 amperes D. C. current flow? 396 

•When using the straight arc, is there increasing loss 
as amperage is increased above 60 D. C? 396 

How may you prove that the straight arc cannot 
use above 60 amperes without waste? 396 to 398 



In what proportions is screen illumination increased 
at different amperages, using the straight arc?.. 399 

Using the straight arc, will an increase over 120 am- 
peres increase screen illumination? 399 

Arc Voltage 

Must a projection arc operate at a certain fixed 
carbon separation, its amount depending upon 
amperage, for maximum results ? 399 

What is meant by "arc voltage" ? 399 

What size and kind of National carbons are recom- 
mended for different amperage? Table No. 24, page 400 

Does a projection arc of given amperage always 
operate at its point of highest efficiency with one 
fixed distance of carbon separation? 399 


Describe a simple, wet battery cell.... 6 

What is the voltage of a single, simple battery 

cell? 7 

How much amperage will a simple, wet battery cell 

produce ? 7 

What are batteries primarily intended for? . . . 7 

Which is the positive and which the negative of a 

wet battery ? 7 

What should be the comparative area of the copper 

(+) and the zinc ( — ) metals of a wet battery 

cell ? 7 

Bell Wiring 

Describe a simple bell or buzzer circuit? 457 

What kind of battery is best for theatre work? 458 

What kind of wires should be used for bell cir- 
cuits ? . . . . 458 

Should a special insulation be used in wet places?.. 458 
Is it permissible to use iron staples to hold more than 

one bell circuit wire ? 458 

What will be the effect of a short circuit in a bell 

circuit ? • 458 

Should bell wire spices be soldered and insu- 
lated? 458,459 

Describe a series, parallel (multiple) and series- 
multiple battery connection, what is the effect of 

each? 459 

Describe the "three-wire" bell circuit system. .459 to 461 
What is the rule to be followed in installing a 3- 
wire bell circuit ? 461 



How would you proceed to install one or two fire 
bells in such way that both bells would be rung 
by any one of several differently located but- 
tons ? 461, 462 

Describe the installation of an electric programme 
board 462 


From whence does practically all available illumina- 
tion from the ordinary arc come ? 374, 387 

Does all illumination emanate from the crater when 
White Flame carbons are used, with A. C. at the 
arc? 374 

Describe the process of manufacture of carbons 
roughly 374, 375 

Purpose of Core 

Why is a core placed in positive carbons? 375 

Why is it that although you bring the carbons very 
close without current "jumping the gap," once they 
have brought into contact and current flow started, 
they then may be separated half an inch or more 

without breaking the arc? 375, 376 

When is a cored negative necessary to good results ? 376 

Is it now good policy to use a solid negative? 377 

What is the "Orotip" carbon? 377 

Carbon Diameter Important 

Is the carbon diameter of large importance? 377 

If the carbon be too small, what will result? 377 

What is the practical results of "penciling" or "need- 
ling"? 377, 378 

What is the practical effect of using a too large 
diameter carbon ? 378 

Crater Brilliancy 

What do authorities say about the brilliancy of 
crater floors ? 378 

Is the highest incandescence — volatilization tempera- 
ture — only had at a very thin layer of the crater 
floor? 378 

Name Them 

Upon what two main factors is the efficiency in light 
production in a projection arc dependent? .378, 379, 394 

What resistance do the various elements of an elec- 
tric arc offer ? 393 

Has the carbon arc the greatest brightness per unit 
area of any light source known ? 379 



Is the total light available from an arc crater in pro- 
portion to the area of its crater floor? 394 

In what channels is the energy developed in an 
ordinary arc dissipated ? 379 

What factors control in the selection of materials 
for the making of projector carbons ? 379 

Name one important asset of the projection arc 380 

Carbon Diameter 

What effect has carbon diameter on light produc- 
tion? . 380 

Name the advantages the small diameter negative 
possesses 380 

A Poor Light Source and Economy 

Is it true economy to use a light source of inferior 
quality? 381 

Is it of importance that the projectionist under- 
stand the effect of amperage change on light pro- 
duction ? 381 

What is the relation of crater area to candle power? 
Figs. 109-110 381 

With carbons of proper diameter for current used, 
does crater area increase more rapidly than the 
current increases ? 381 

What is the positive crater area at various amper- 
ages? Table 19 382 

When amperage is doubled, how much is candle 
power increased ? 383 

Light Source Area and Optical System 

What happens when the light source area is too 

great for the optical system in use? 383 

Examine and study Fig. 112 and accompanying text. 384 
What relation does screen illumination bear to am- 
perage increase ? Table 18 385 

White Flame Projection Carbons 

What advantages do White Flame carbons (for use 
with A. C.) present when compared with ordinary 
cored carbons ? 385 

Name one large advantage in White Flame carbons 
from the viewpoint of the projectionist 385 

At what maximum amperage may white flame car- 
bons be used to advantage ? 386 

At what range of amperage is the crater formation 
of the white flame carbon far better than that of 
the ordinary carbon ? 386 



What is the maximum amperage at which ordinary 
cored carbons may be used advantageously on 
A. C? 4- 386 

What does Fig. 113, page 387, show? 386 

Conclusion as to Maximum Load for Carbons 

What conclusion may be reached concerning the 
loading of carbons for maximum results in pro- 
jection work ? 386 

Should carbons be carefully inspected before using? 387 

Should the cores be especially examined for imper- 
fections ? 388 

What is the presumed reason for hard spot in car- 
bons ? 388 

Care of Carbons 

What care should carbons have? 389 

What is a "carbon economizer"? 389 

Carbons, Setting The 

What item is of prime importance in projection 
light production from an electric arc? 402 

What is the primary purpose of the projection arc 
and can that purpose be accomplished if the crater 
does not face the collector lens as nearly square 

as possible ? 402, 403 

Crater Angle 

What should be the position or angle of the crater 
floor with relation to the face of the collector 
lens, or floor of the lamphouse ? 403 

By what is the angle of the crater with relation to 
the lens determined or controlled? 403, 405 

Considering the ordinary arc, must the carbons be 
in exact line with each other as viewed through 
the condenser opening ? 403 

How would you proceed to test the alignment, side- 
wise, of the carbons of a straight arc lamp? 404 

What would be the practical result of carbons which 
were not in true alignment sidewise? 404 

How may failure of carbons to line with each other 
sidewise be remedied? 405 

Before testing for side alignment what must you be 
certain of? , 405 

Maintaining Crater Angle 

How would you prepare to maintain a constantly 
correct crater angle ? 408 

How would determine whether or not your crater 
angle was correct? 408 to 412 



Coloring Incandescent Lamps 

How may incandescent lamps be colored? 470 

Condenser, The 

(Cinephor Condenser, see page 924, Vol. II.) 

What is the purpose or function of the condenser? 

126, 160, 171 
Types of Condenser 

What three types of condenser are used for mo- 
tion picture projection in the United States and 
Canada? 164, 168 

Of what is the piano convex condenser composed?.. 168 

How far apart should the lenses of any type of con- 
denser be spaced ? 168, 169 

What is the "collector" lens of a condenser? 164 

What is meant by the "converging" lens of a con- 
denser? 164 

Of what elements do the various types of condensers 
consist ? 168 

Is the converging lens of a Cinephor parabolic con- 
denser always of one focal length? 168 

Does the focal length of the collector lens of Cine- 
phor condensers vary in different cases? 168 

What is a meniscus bi-convex condenser composed? 168 

What advantage does a meniscus bi-convex conden- 
ser possess over a piano convex condenser? 168 

Why are the convex (rounded) sides of piano con- 
vex lenses always placed next each other? 169 

For what fault is the Cinephor parabolic condenser 
largely corrected ? 169 

What chief advantage does the Cinephor condenser 
possess? 169 

Is there any longer an advantage in using menis- 
cus bi-convex condensers? 169 

Importance of Good Lens Surface 

Is it essential that the condenser lenses have a 
perfectly true, well polished surface ? 167 

What proof do figures 38 and 39 offer that true 
condenser lens surfaces are important? 187, 188 

What will happen if the surface of a condenser con- 
verging lens be not perfectly true? 167, 168 

How may we prove that a condenser projects for- 
ward to the spot a pencil of light from each pin 
point of its surface, directing it to its appointed 
place in the spot ? 167 



Absorption Loss Small 

What is the loss through absorption of light in pass- 
ing through clear, colorless glass of good grade? 

145, 164 

What is the added percentage of light absorption 
when the condenser lens is discolored? 164 

Why is it impossible to focus a condenser beam to 
a point? 136 

Heavy Loss Possible 

What possibility for serious light loss is involved 
in the distance of light source from face of col- 
lector lens ? 162 

Supposing the face of a collector lens to be 2}4'' 
from the light source. What increase in its diam- 
eter would be necessary to cause it to pick up an 
equal amount of light if it be moved back to 
3^"? 162 

Does the condition shown in Fig. 36H, page 162 hold 
entirely true ? 163 

What is the first task in connection with the ad- 
justment of a motion picture optical train? 163 

Lens Chart Basis 

What item forms the basis of all properly con- 
structed charts designed to cover the selection 
and adjustment of the elements of a motion pic- 
ture projector? 163 

Is it practical for the projectionist himself to deter- 
mine what the minimum distance of light source 
from collector lens is under any given condition? 164 

Is there such a thing as a 3-lens condenser in use? 164 

Ghost Zone in Beam 

(See "Projection Optics, Practical") 

Color in Condenser 

Should the projectionist avoid color in purchasing 
condenser lenses, and how should he examine a 
lens for color ? 165 

Is it possible that certain color tints may be ad- 
vantageous in a condenser, notwithstanding the loss 
involved? 165 

Unless it be decided, by careful experiment, that a 
certain color is of advantage, what should be 
done with lenses which develop color with use ? . . 165 



Pitting of Collector Lens 

What do you know about pitting of the face of the 
collector lens and its practical effect? 165, 166 

Is it known what actual light loss a pitted collector 
lens may cause, and what may we fairly assume 
in that connection? 166 

Why is a sharp edge condenser lens impracticable ? 166 

Lens Edge Thickness 

What should the thickness of the edge of a piano 
convex condensing lens be? 166 

What advantage is there in exactness in condensing 
lens diameters and edge thickness? 166 

Reflection Loss 

How much light loss through reflection is inherent 
in a 2-lens condenser, and how much may that in- 
herent loss be increased if the lens surface be not 
clean ? 168 

What types of condenser are available for projectors 
using an arc ? 168 

Spacing the Condenser 1 Lenses 

How should your condenser lenses be spaced — how 
far apart ? 169 

Why may the lenses not be placed in actual physical 
contact with each other ? 169 

Upon what condenser lens spacing are lens tables 
computed? .169, 170 

Light Loss Between Condensers 

Does the collector lens send forward a parallel 
beam of light? 170 

Is it not an obvious fact that spacing your con- 
denser lenses further apart than is necessary sets 
up added light loss ? 170 

How may the projectionist convince himself that the 
beam of light from the collector lens more than 
covers the converging lens? 170 

Condenser Mounts 

Name the points of excellence in a condenser mount? 170 
Should the condenser mount be so constructed that 
the projectionist may alter the spacing of the lens 

mounts at will ? 171 

Is it better to have the condenser mount inside or 
outside the lamphouse? 171 



Condenser Breakage 

To what various things is condenser breakage usually 
traceable ? 171 

What the Condenser Does 

What is the office or function of the condenser? 

126, 160, 171 

The crater of the ordinary projection arc is not in 
focus at any one plane. Why is this so? 171 

Does the core of an arc crater offer a different de- 
gree of luminosity than the rest of the crater 
floor? . 171 

In what w r ay is the crater image broken up and 
prevented from focusing at one plane ? 172 

Is the light shown as wasted — light which passes 
through the aperture but not through the pro- 
jection lens — worse than merely wasted? 

Fig. 59 and Pages 204, 205 
Condenser Diameter 

Should projectionists be especially interested in the 
matter of condenser diameter ? 205 

Is there any advantage in using a large diameter 
uncorrected condenser ? 205 

What is meant by the "Aerial Image" of the con- 
denser ? 205 

Brinkert Apparatus 

(See Volume II) 

Current Rectification 

(See Volume II) 

Dowsers, Automatic 

(See Volume II) 

Electrical Action 

What basic knowledge is necessary to an under- 
standing of electrical action ? 3 

Why is it necessary that the projectionist possess a 
good knowledge of electrical action ? 3 

What will happen if the projectionist lacks a good 
knowledge of electrical action ? 3 

Are electricity and magnetism essentially the same 
thing ? 

Are electricity and magnetism in any way related 
to each other? 3 



What is Electricity? 

Can you tell what electricity is? 3, 52 

Tell how to proceed in order to cause electricity to 
perform work ? 3 

How many poles has a generator or dynamo? 4 

How is power conveyed from an electric generator 
to distant points? 4 

Would or would not the effect be exactly the same 
were you to connect a lamp or motor to your 
theatre wires, or directly to the poles of the gen- 
erator? 4 

What is meant by a "dead" and a "live" wire?.... 4 

Steam loses its pressure by expanding, usually 
through escape into the open air. In what way 
does electricity reduce or lose its pressure — vol- 
tage? 4, 52 

To what may electric voltage be compared? 4, 52, 60 

What Is Polarity? 

What is the element we call "polarity"? 4, 50 

In what is difference in polarity (electrical pressure) 
measured ? 5 

In what way does steam perform work in a steam 
engine? 5 

When steam performs work what is it that is con- 
sumed? 5 

Illustrate the fact that power (pressure) only is con- 
sumed when work is performed, and not the med- 
ium through which it works, using some power 
storage medium other than steam and electricity 5 

Can you see or weigh electricity? 5, 51 

Similar in Action to Water or Steam 

To what is electricity very similar to in its action? 5, 50 

Why is power generated when electricity passes 
from positive to negative ? 5 

Does electricity seek to escape from power lines 
to the earth? 5, 51 

Has the positive wire of an electric circuit an 
electrical affinity for anything else except the 
negative of the same power source ? 6, 51 

Were you to make electrical contact between the 
positive of one electrical power source and the 
negative of another electrical power source not 
electrically attached to the first, what would 
happen? 6 

Assuming an electrical power source and all its 
connections to be thoroughly and completely in- 
sulated from earth, what would happen were its 
positive wire to fall to wet earth? 6 



Direct and Alternating Current 

Describe the difference between direct and alternat- 
ing current 13 

Why is alternating current used more than direct 

current? 13 


Explain why it is possible to "carry" a greater 
horse power of electric energy over a wire of given 
size at high than at low voltage 13, 14 

How many alternations per second in 25, in 60 and 
in 120 cycle current? 15 

Is there any limit to the number of cycles current 
may have? 15 

What cycle is most commonly used, and why is it 
used most commonly ? 15 

Why is low cycle current undesirable for lighting 
work ? 15 

Action of Alternating Current 

What does the central, horizontal line represent 

in Figure 4, Page 16? 16 

What does the distance from to 1, from to 2, and 

from to 4 on the horizontal line in Figure 4, 

Page 16, represent ? 16, 17 

What does the height of triangle A, Figure 4, Page 

16, represent ? 16 

Explain, in detail, just what the height of line B, 

Figure 4, Page 16, and its slant to the right means 

16, 17 
What does the whole of triangle A, Figure 4, Page 

16, represent ? 16 

Is the light from an A. C. arc, or other A. C. light 

source continuous as to its brilliancy? 17, 18 

Two and Three Phase Current 

Explain what is meant by a two-phase current 18 

Explain what is meant by a three-phase current... 18 

In what way are two and three phase currents pro- 
duced ? 18 

How is a 2-phase current usually transmitted and 
what are its advantages ? 18 

How many wires does 3-phase usually employ and 
for what is three-phase current ideal? 18 

One wire always is positive and one negative in a 
2-wire A. C. circuit when D. C. is used. What is 
the condition in this respect when A. C. is used?.. 50 

In what respect may the action of electricity be 
compared to that of water under pressure? 50 

To what may voltage be compared? 50, 51 




Suppose you attach a wire half a mile long to either 
pole of a one volt battery, the wires being thor- 
oughly insulated from earth and from each other. 
Would they show one volt pressure at any point 
in their length ? : 51 

Suppose you connect the positive of one battery or 
dynamo to the negative of another similar unity of 
equal voltage, what is such a connection called, and 
what is its affect in voltage ? 50 

Does anything actually pass or flow over or through 
a wire over which electric energy (power) is be- 
ing conveyed ? 52 

Explain, in detail, how work is performed by elec- 
tricity 52 

If we have a circuit charged at 100 volts and use 
ten amperes from it, what effect would it have, 
in work performed, if the voltage or amperage, 
or both, be doubled? 52 

In a steam engine or water motor we may increase 
the power by increasing either the pressure or the 
size of the engine or motor. Does the same thing 
hold true with electricity? 53 

Watts and Horsepower 

What term represents electric power and how is it 
reduced to horsepower? 53 

What is the physical effect of the resistance current 
encounters in passing through a conductor? Note — 
the effect will, unless the circuit be overlooked, be 
too slight for anything by a delicate measuring 
instrument to detect, but it is there nevertheless. 53 

In a projection arc circuit, name the various items 
offering resistance to current flow 54, 57, 393 


(Also see "Resistance, Index.) 

What extremely important factor enters into all 
electrical work ? 60 

Explain the action of water in Fig. 6, Page 61, and 
tell us in what way or ways it resembles the action 
of electric current 61 

When is the normal capacity of a water pipe or 
wire said to have been reached ? 62 

How does increasing or decreasing the diameter 
of a water pipe or wire effect its resistance, assum- 
ing the flow to remain constant? 63,64 

How does increasing the length of a water pipe or 
wire effect its resistance? 63, 64, 68 




Name the various things which will increase resis- 
tance in the case of a water pipe or electric 
circuit 63, 64, 68 

Has the E. M. F. (voltage) any effect on the size of 
wire required to carry a given number of am- 
peres without overload ? 63 

What does effect the amount of electrical energy 
which may be carried by a wire of given size? 14 

Overload Begins 

What is the point where overload begins in an elec- 
trical conductor? 64 

Name two important reasons why an electric circuit 
should never be in any degree overloaded 64 

Do different metals offer different resistance to elec- 
trical current ? 65 

Does the resistance of all metals used in electrical 
work increase as their temperature increases?.. 65 

Can You Name It? 

Name one thing used to conduct current, under some 
conditions, the resistance of which decreases as 
its temperature increases 65 

Name other things which have less resistance when 
hot than when cold 66 

To what is the increase or decrease of resistance in 
electric conductors due to changes in tempera- 
ture proportioned ? 66 

What is "Normal Temperature" and for what is it 
used? 66 

Electrical Terms 

What does the "Watt" represent, and how is its 

value ascertained by calculation ? 53 

What does the term volt represent. To what may it 

be compared? 50, 51 

What is meant by the term "Cycle" ? 26, 75 

What is meant by the term "Frequency"? 29 

What is meant by the term "Alternation"? 15 

What is meant by "2-phase current" ? 18 

What is meant by single-phase and 3-phase cur- 
rent? 18 

Is there any difference between "Potential" and 

"Polarity" ? 50 

What is the meaning of the term "ampere"? With 

what familiar thing may it be compared? 51 

What does the term "Ohm" represent ? 53 



Emergency light Circuits 

Where should the circuit carrying the various emer- 
gency light circuits be connected to the main house 

service wires ? 103 

Where should the switches controlling emergency 

light circuits be located? 103, 104 

What lights are termed "emergency lights"? 104 

Wher6 should emergency circuit fuses be located?.. 120 
What special fuses should be used for emergency 
lights? 120 


Projection Room . 319 


(See pages 234 to 239; also page 244) 

Film, The 

What are the dimensions of film ? 268 

Roughly describe the process of film making. .268, 269 
Is it important to goo dresults in projection that 
the film stock be of uniform, unvarying thick- 
ness ? 269 

What standard dimensions have been adopted by 
the Society of Motion Picture Engineers? 

Fig. 77, Page 270 
What is the maximum shrinkage of film per foot?.. 269 

Damage to Film 

When film is new what part of it is especially sus- 
ceptible to damage ? 270 

Name some of the various ways in which film re- 
ceives damage 270, 271, 333 

What is "pulling down" and what damage is caused 

to film by it? 271, 333 

• What various things work greatest damage to 

sprocket holes ? 271 

Does the unintelligent, careless handling of film by 
users and others do great damage ? 272 

Should film repairing, in the theatre, be done by 
any other one than a thoroughly competent man? 272 

To what things is damage to film in process of pro- 
jection due ? 272 

Is the sending out of film by exchanges in poor me- 
chanical condition an outrage ? 272 

What is likely to happen if sprocket holes be pooriy 
matched in splicing the film ? 273 



Emulsion Deposit 

By what may the deposit of emulsion on projection 
tension shoes be increased ? 273 

Film Waxing 

What is the best method yet found of preventing 
emulsion of new film f**om depositing on tension 
shoes of projector? 273 

Mending Film 

Are poorly made splices a source of enormous dam- 
age to film? 274, 275 

Are hand-made splices a source of great annoyance 
and trouble ? 275 

Name some of the projection faults due to poorly 
made splices 275, 276 

Is it necessary to perfect splicing that a splicer of 
some sort be used ? 277 

What should be the width of a splice ? 277 

Is there any practical advantage in a very narrow 
splice in the positive film? 277 

Should there be a full-hole sprocket hole in the 
splice? 278 

How to Make a Splice 

Describe the process of making a film splice.. 278, 279 
Is it of especial importance that the emulsion be 
scraped off the stub-end perfectly in making 

a splice ? ■ 278 

Is it of especial importance that emulsion be removed 
cleanly from around sprocket holes in making 

splices ? 279 

Is it of importance that you scrape the emulsion to a 

straight line in making splices ? 279 

Does use of too much cement cause trouble? 279 

Describe the ideal cement bottle. See cut page.... 271 
Name some excellent film cement formulas.... 280, 281 
Is a good film splicer essential to good splicing? 278 

Film Inspection 

What film damage is it the duty of the projectionist 
to repair ? 286 

Is the theatre paying for thorough film inspection 
and repair ? 286 

Has the projectionist who receives films which have 
not been inspected and repaired a right to ex- 
pect pay for doing that work ? 287 

Is perfect projection possible with film not in good 
mechanical condition? 287 



Do exchanges, or some of them, deliberately attempt 
to force the projectionist to do their inspection 
and repair work free of cost? 287 

Do film inspectors themselves often abuse film? 288 

Does the "inspection" in many exchanges detect 
anything except the very worst faults ? 288 

Is there any excuse for the utterly wretched con- 
dition in which some exchanges send out films?.. 288 

Should projectionists demand overtime pay for in- 
specting and repairing films received in bad con- 
dition ? 288 


Does improper rewinding of film cause great damage 

thereto ? 270 and 332 

Does the process known as "pulling down" cause 

great damage? .' 271,333 

Is rewinding film at high speed bad practice? 333 

Should the motor driven rewinder be geared down 

in speed ? 333 

What should be the maximum rewinding speed?.... 333 
Aside from decrease in damage to film induced by 

slow rewinding, what other thing is in its favor? 

333 f 334 
Is it highly essential that the rewinder elements be 

in alignment with each other? 332 

Should a good film splicer be a part of the rewinder 

table equipment ? 227 

Where Films Should Be Kept 

In what location in the projection room should 
films be kept, and why ? 288 

Moistening Dry Film 

Describe the method of re-moistening dry films 289 

How to Inspect Film 

How would you proceed to inspect films? 290 

Removing Emulsion 

How may emulsion be removed from film? 290 

Cleaning Film 

Tell us what you know about cleaning film 290, 291 

Measuring Film 

How may film be measured as to its footage? 292 



Fire Hazard — Safety 

To what ought authorities to pay close attention in 
the matter of removing possibility of tire panic?. 247 

How may screen or other fabric be readily fire 
proofed ?/ 249 

Port Sizes 

Does limiting the size of projection room ports serve 
any purpose of safety to the audience ? 312 

In what direction might the authorities direct their 
energies to better purpose than by setting up 
foolish limitations of projection room port sizes? 312 

From what should port fire shutters be made?.... 312 

Real Protection to Audience 

What offers the only real protection against fire 
damage outside the projection room and against 
fire panic on the part of an audience ? 313 

If the projection room be properly equipped with 
fire shutters properly fused and hung, is there 
any danger to an audience from a fire in the pro- 
jection room? 313 

Inform Audiences 

Ought audiences to be made acquainted with the 
fact that the theatre projection room is fire proof, 
and that a fire therein holds no danger to them 
except through their own panic ? 313 

Is it entirely possible to prevent audiences from 
knowing there is a fire when one occurs in the 
projection room? 313 

Port Shutter Fuses 

How should the port shutters be hung and fused?.. 314 
Do the authorities for the most part err in the mat- 
ter of locating port shutter fuses? 316 

Why should port fire shutter grooves be padded at 
the bottom? 316 

Generators, Electric 

(See "Current Rectification" for detailed instructions on 
motor generators.) 

How many poles may a generator (dynamo) have? 4 
Would or would not the effect be exactly the same 
were you to connect a lamp or motor to your 
theatre wires, or directly to the poles of the 
generator? 4 


Why is it imperative that the projectionist have a 
good practical working knowledge of generators? 6 

Describe a simple, wet battery cell 6 

In a wet battery cell which is negative and which 
positive ? 7 

Upon what fundamental law does the armature of a 
dynamo depend for its action ? 7 

Armature Action 

Explain the electrical action of the elementary gen- 
erator armature shown diagramatically in figure 
1 page 7 7 

All Armatures Generate A. C. 

What kind of current does every generator arma- 
ture generate ? : 8 

Explain w T hy dynamo armatures generate alter- 
nating current 8 


In what way is direct current obtained from an arm- 
ature generating alternating current ? 9 

What is the changing of A. C. into D. C. in a dynamo 
armature called ? 9 

Describe the "commutator of a dynamo armature"? 

9, 10 

Coils Inter-Connected 

Are all the coils of a dynamo armature inter-con- 
nected? 11 

Name the various parts of a single dynamo 11 

Upon what does the amount of E. M. F. a dynamo 

will generate depend ? 11 

What is meant by a "permanent magnet"? 12 

What is meant by "residual magnetism"? 12 

What important part does residual magnetism play 
in the electric dynamo ? 12 

Increasing Magnet Strength 

What is the effect of causing an electric current to 
flow through a wire coiled around an electric 
magnet ? 12 

To what extent may the strength of an electric 
magnet be increased by causing current to flow 
through a wire coiled around it? 12 

What is meant by "saturation" as applied to an elec- 
tric magnet ? 12 

Explain how the armature of a self-exciting dynamo 
starts generating E.M.F., and how the strength 
of that E. M. F. is built up to maximum 12 



Do generators (dynamos) have more than two pole 
pieces and why are more than two pole pieces us- 
ually employed in dynamo construction? 13 

Which way does current generated by a dynamo 
flow? 13 

Is every electric generator (dynamo) built to oper- 
ate at a certain voltage ? 51 

Voltage (E. M. F.) will act between two things, and 
two things only. What are they ? 51 


What is the purpose of the fuse — the reason for its 

use? 107 

W r hat is the composition of fuse wire? 107 

What is the practical operation of a fuse ? 108 

In what way do fuses protect an electric circuit and 
the apparatus attached thereto ? 108 

Link Fuses Allowed 

Is the use of raw fuse wire permitted in a theatre 
projection room ? 108 

What types of fuse is the projectionist likely to en- 
counter in his work, and what are the only types 
permitted for general theatre work? 109 

What is a "Cartridge Fuse"? Describe it in detail. 

109, 110 

What is a "Plug Fuse"? Describe it in detail 110, 111 

W'hat is a "Link Fuse"? Describe it in detail Ill 

Underwriter's Requirements 

What various things do the Underwriter's rules re- 
quire with relation to cartridge fuses? 110 

What are the required dimensions for cartridge 
fuses of various voltage and amperage capacities? 

112, 113 

What is the maximum capacity of plug fuses in their 
regular and special forms? Ill 

Why do some authorities require the use of link 
fuses for projector light source circuits? Ill 

Fuse Boosting Reprehensible and Dangerous 

What is meant by "boosting" a fuse ? Ill 

Does boosting fuses usually leave the circuit and 
apparatus attached thereto without any protection 

at all? Ill 

What serious damage may be caused by fuse boosting? Ill 
What should be done to the projectionist or machine 

operator caught boosting fuses ? 114 

What should govern in the fusing of ordinary circuits? 114 



Fusing Motor Circuits 

What two things must govern in fusing motor 
circuits ? 114 

What provision for excess fuse capacity do Un- 
derwriter's rules make in the matter of motor 
circuits ? 114 

What must you be certain of with regard to the 
wires if you fuse a motor circuit above motor 
capacity? 114 

Refilling Old Fuses 

Is it possible to refill old fuses? 114 

What should be done with old fuses, and why? 114 

Projection Circuit Fuses 

Why is it advisable to fuse projection arc circuits 
higher than the normal amperage flow and what 
should the excess fusing be for different amperage? 

114, 115 

What allowance must be made when fusing on prim- 
ary side of a motor-generator, mercury are rec- 
tifier or transformer? 115, 116 

Double Fusing Motor Generator, Etc. 

When a motor generator, rotary converter, mercury 
arc rectifier or economizer is used, ought there be 
fuses on both primary and secondary circuits? 115 

What allowance ought to be made above normal am- 
perage when fusing on secondary side of motor 
generator, etc., set? 115 

For the purpose of using on primary side, how would 
you calculate amperage on primary side if secon- 
dary amperage be known ? 116 

To obtain accurate results in calculation called for 
in preceding question, what would have to be 
done? 116 

What would be necessary if generator voltage be 
higher than arc voltage ? 116 

Faulty Circuit 

If a fuse blows and a new one you install blows 
immediately, what does it indicate ? 117 

Faulty Contact 

If a fuse blows and the new one you install blows 
after a time, and there seems no fault in the cir- 
cuit or any overload, what would you look for?.. 117 

Name one thing which may cause sudden overload, 
and what visible cause of the trouble will there 
be? 117 

Will loose, dirty contacts cause a fuse to blow?.. 118 



Emergency Fuses 

How may you protect a circuit reasonably well for a 
short time you arc short of fuses and one of its 
fuses is still good ? 116, 117 

What substitute may be used for fuses in an 
emergency ? 117 

Are copper wire fuses reliable ? 117 

Where may you always secure small wires from 
which a copper fuse of any desired capacity may 
be made? .-. 117 

How many strands from an asbestos covered wire, 
such as is used for projector arc lamps, would 
make approximately a 40 ampere fuse? 117 

Testing- Fuses 

Describe a simple, easily constructed fuse tester.. 118 
Why should a fuse tester be a part of your pro- 
jection room equipment? 118 

How are fuses marked as to their voltage and ' 
capacity? 118 

Where Fuses Should Be 

Where should fuses be installed? 119 

What current should the main fuses of a theatre 

carry? 119 

What should the main projection room fuses carry? 119 

Emergency Light Fuses 

Where should the fuses for emergency light cir- 
cuits be located ? -. 120 

What special fuses, other than the regular circuit 
fuses, should be installed for emergency lights? 120 

In General 

Must every circuit, no matter how small, be protected 
by fuses ? 120 

Does the blowing of the fuse of a projection arc 
circuit necessarily mean that anything is wrong 
with the circuit ? 120 

Double Fusing 

How may you have a new set of perfect fuses ready 
for instant use when a fuse blows ? 120 

Glare Spots 237 

Ground and Testing for Same 

Is there any such thing as a "ground" except as the 
earth offers a path to opposite polarity ?... .5, 51, 352 



What is meant by one part of an electrical device 
being "grounded" to another part of the same de- 
vice ? 352 

Does it follow that because there is a "ground" there 
is current flow ? 353, 354 

Is the neutral of an Edison system always grounded 
to earth? 353 

Explain the action of a true ground, as illustrated in 
Fig. 101 353 

Are there two kinds of 3-wire systems, one grounded 
and one not? 354 

Why is Edison 3-wire neutral grounded? 354 

With a grounded system will the test lamp show 
light from neutral to ground ? 355 

Why may fuses blow when arc is struck if rheostat 
be on neutral of a grounded 3-wire system? 355 

Testing for Grounds 

What various things may be used for testing for 
grounds ? 356 

What is the practical testing tool for the projec- 
tionist ? 356 

How would vou make a test lamp to be used on a 
110-220 volt 3-wire system? .' 356 

Describe the method of testing for ground with 
permanent ground and lamp 356, 357 

Should projector ground wire be disconnected before 
testing lamp for grounds ? 357 

How would you test for grounds with battery? 357 

How would you test for grounds with a magneto?.. 357 

Locating Grounded: Rheostat Coils 

See Rheostatic Resistance. 

Grounding the Projector 

Should the projector lamphouse, mechanism and 
frame be permanently grounded ? 360 

Is it necessary that the metal of the projection room 
be grounded ? 360 

Effects of Grounding 

What is the practical effect of a ground in the pro- 
jector lamp ? 360 

Should the projector lamp and its circuits be tested 
for ground every day ? 361 

Should the projectionist dust off the insulation of 
his lamps every day ? 361, 370 



Heating, Maximum Permissible Temperature 

(See Volume II) 

High Intensity Lamp 

(See Volume II) 


What is the purpose of insulation — what is it for?.. 80 
What class of substances do we call "insulating 

materials" ? 80 

Are materials other than metals and carbons con- 
ductors of electricity? , 80 

No Absolute Non-Conductor 

Is there any known material which is an absolute 
non-conductor of electricity ? 80 

What materials are commonly considered as non- 
conductive of electricity? 80 

Name some of the materials commonly used for pur- 
poses of insulation 80 

Why are rubber covered (R. C.) wires rated at less 
amperage capacity than wires having other types 
of insulation? 83 

Rubber Covered 

Of what does rubber covered insulation (R. C.) con- 
sist? 80 

Why is it necessary to coat wires which are to re- 
ceive R. C. insulation with tin ? 281 

With increase in the power of one element of elec- 
tricity it becomes necessary to increase the insula- 
tion. What element is that ? 81 

Weather Proof 

Of what does "Weatherproof" insulation consist?.. 82 

Where is it permissible to use weather-proof insu- 
lation? 81 

In what classes of work is it necessary to use R. C. 
insulation ? 81 

Is it permissible to use any other than R. C. in- 
sulation in conduit ? 83 

Under what conditions is it forbidden to use R. C. 
insulation? 83 

Name the two types of weather-proof insulation. . . 82 



Slow Burning 

Of what does "Slow-burning Weather-proof Insula- 
tion" consist? 82 

May fire-proof insulation be used in the open — out of 
doors ? 82 

What is the difference between "Slow-burning Wea- 
ther-proof ' and "Weather-proof* insulation?,,,, 82 

Intermittent Speed, Measuring of 

(See Vol. II, page 659) 


Quote the law which deals with light intensity at 
different distances from its source 125 

How may you demonstrate the correctness of the 
law referred to in previous question? 161 

Explain the action of this law as exemplified in 
Figure 27, Page 126. 126 

Of what is the motion picture projector optical 
train composed ? 125, 126, 160 

What is the first element of the optical train and 
what is its function ? 126, 160, 171 

The second lens element of the projector optical 
train is the projection lens. What is its func- 
tion? 126, 160, 171 

Why will line F A, Figure 27, Page 125, be perpen- 
dicular to both surfaces of the lens ? 126 

Light Refraction by Lens 

Why would a ray of light represented by line F A, 
Figure 27, Page 125, not be refracted at all by the 
lens, and why would a minimum percentage of 
its light be reflected by the surface of the lens? 

126, 141 

Explain the refraction of the rays represented by 
lines F C and F G, Figure 27, Page 125.... 126 

Are rays refracted by a lens except at its surface?.. 131 

Upon what law is the action of lenses based? 127 

Upon what factors does the amount of bending light 
rays will receive in entering or leaving a lens ? . . 128 

Explain the action of light rays as shown in Fig. 
28, Page 128 .•••:••■ 128 

For all practical purposes what may the projection- 
ist assume that the amount of bending or refraction 
light rays will receive in passing through a lens 
will depend? 129 

Where may the student find explanations of why 
light is refracted in passing through a lens ? 129 



Spherical Aberration 

Explain what is meant by Spherical Aberration in a 
lens, and explain its action, illustrating by dia- 
gram 129, 130 

What is that quality in a lens which produces 
chromatic aberration? 130 

Lens Correction 

How are lenses corrected for chromatic and spheri- 
cal aberrations ? 130 

What must the student get in order to understand 
light action through lenses ? 131 

From the optical viewpoint what does each pin- 
point of the surface of a lens present? 131 

Why does every pin-point on the surface of a lens 
differ optically from every other pin-point on the 
same lens? 131 

Fundamental Facts 

What fundamental fact should the student get fixed 
in the mind? 131 

Why is it imperatively necessary that the surfaces of 
lenses be optically true and perfect in their curva- 
ture? 131 

Name two faults which all uncorrected lenses have, 132 

Focal Length 

What is meant by the focal length as applies to a 

simple lens? 134 

What determines the focal length of a simple lens?.. 134 
How is the curvature of a simple lens determined?.. 134 
Were you to cut a polished glass ball in half and 
polish the flat sides, what would you have as a 
result? 135 

Lens Image Has Area 

Will a lens focus any object having area to a pin- 
point ? 135, 136 

Why is it impossible for the condenser to focus the 
light beam to a pin-point ? 136 

What is the maximum free aperture of projection 
lenses? 139 

Ordering Lenses 

What range of focal length of projection lenses are 
carried in stock by manufacturers? 140 

In ordering projection lenses what data should you 
send with your order ? 139 



Reflection Light Loss 

What is the percentage of light reflected from the 
polished surface of each lens? 141 

What effect has an unclean lens surface on the 
amount of light it will reflect, therefore waste?.. 141 

Upon what two elements does the amount of light 
pend? 141, 142 

When what two elements are reduced to their lowest 
factor will loss of light by reflection be reduced 
to its minimum ? 142 

What is meant by the "Angle of Incidence"? 129, 142, 222 

What is meant by "Ref ractive Index" ? 40 


What is the formula representing the loss of light in 
entering or passing the surface of a lens ? 142, 143 

What is the variation in light loss at the surface of 
a lens with various angles of incidence? 143 

What are the refractive indexes of Crown and Flint 
glass ? 143 

Absorption of Light 

Upon what does the loss of light through absorp- 
tion in passing through the body of the lens de- 
pend? 145, 164 

What is the range of angles of incidence of light 
upon projection lens surfaces ? 144 

Does the reflection loss within the angles to 30 
(see previous question) amount to much? 144 

Is the loss of light through reflection at the cemented 
surfaces of the front factor of a projection equal 
to that at the surfaces exposed to air? 144 

What is the ratio of absorption (loss) in glass of 
varying grades ? 145, 164 

What is the total loss in a flint lens of 1.61 refrac- 
tive index ? 145 

What is the total approximate loss in a cemented 
crown and flint lens? 145 

What may we safely assume the loss per single lens, 
or per two cemented lenses to be ? 146 

Is the amount of light passed by all similar lenses 
the same? 146 

Image Formation 

After examining Figure 30, Page 132, explain the 
process by which a lens forms an image 133 

What are the points upon the object from which the 
rays emanate and the points upon the image where 
they are focused called ? 133 



If one conjugate foci point be brought nearer to or 
moved further from the lens, what will be the effect 
upon the other ? 133 

If object X, Figure 30, be moved very close to the 
lens, w T hat will be the result as to rays leaving 
the lens? . 133 

What law is brought into action when the projec- 
tion lens is moved forward or back to "focus" the 
picture ? 133, 134 

If the object (film) be too close to or too far away 
from the film, why will the rays not focus a clear 
image at the screen ? 134, 146, 149 

What are the two conjugate foci points of a projec- 
tion lens? 149 

Projection Lens 

What optical elements has a projection lens? 136 

What is meant by the "front factor" and the "back 

factor" of a projection lens ? 136 

What is used to cement the lenses of the front fac- 
tor together 136 

Of what does the front factor of a projection lens 

consist ? 136, 137 

May the balsam with which the front factor lenses 

are cemented together melt and run out? 137 

Is it possible to correct keystone distortion on the 

screen by means of specially ground lenses? 137 

What care should the interior of the projection lens 

barrel have ? 138 

What damaging effect may appear if the paint be 

permitted to wear off the interior of the projection 

lens barrel? 138 

What should be used to paint the projection lens 

barrel interior ? 138 

In reassembling the individual lenses of a projection 

lens, how should they be clamped in their mounts? 138 

Dirty Lenses 

What is a very important factor in the performance 
of lenses? 146 

What is the result upon the screen image of scummy, 
dust-covered lens surfaces? 145 

Will a dirty, finger marked lens project a good pic- 
ture? 137, 146 

Cleaning Lenses 

Is it essential to best results that lenses be kept 

perfectly clean ? 137 

What effect on the screen image has a dirty lens?.. 146 



What is the effect of oil or finger marks on the 
projection lens? 137 

What is the effect of an even distribution of dust on 
the surface of the lenses ? 137 

Should the surface of the lenses be highly polished 
after each cleaning? 137 

What simple, cheap preparation will serve well for 
lens cleanng ? 137 

What sort of cloth or skin should be used for lens 
cleaning ? 137 

What should the projectionist do to his lenses every- 
day? 137, 138 

Should the projection lens be dissembled for clean- 
ing? 138 

What care should be exercised in disassembling the 
projection lens and reassembling it again? 138, 139 

What may you take for a guiding rule in reassem- 
bling the lenses of a projection lens? » . . . 138 

What care should the lens barrel interior have? 138 

What harm may result from paint wearing off the 
interior of the projection lens barrel? 138 

What should be used to coat the projection lens bar- 
rel interior ? 138 

How should the lenses of the projection lens be 
clamped in their individual mounts ? 138 

Large Diameter Lens Objectionable 161 

Measuring* Lenses 

Is it necessary that the projectionist be able to him- 
self determine the focal length of a lens ? 151 

Can condenser lenses be measured accurately by the 
projectionist ? 151 

Describe two methods of measuring the focal length 
of a condenser lens 151» 153 

What is essential to reasonable accuracy in meas- 
uring a condenser lens ? 151 

How would you measure the focal length of a bi- 
convex lens ? 152 

What two different measurements of focal length of 
projection lenses are important to the projection- 
ist? 152 

What is the "Working Distance" of a projection 
lens? 152 

What is the "Equivalent Focus" (E. F.) of a projec- 
tion lens equal to ? 152 

Why is the working distance of the projection lens 
of great importance to the projectionist? 153 



How would you measure the equivalent focus of a 
projection lens? 152 

How would you measure the equivalent focus of a 
projection lens? 152 

How may you proceed to measure the focal length 
of any simple lens, or the E. F. of any compound 
lens? 153 

How may you determine the focal length projection 
lens necessary to project a given size picture at a 
given distance? 154, 155, 156 

Lens Quality- 
Is it good policy to purchase high grade projection 

lenses ? 156 

Dimensions of various projection lenses may be had 
from figures 36E, 36EE, 36F, 36FF, 36FFF, 36G 
and 36GG. 
Dimensions of other lenses may be had direct from 

the manufacturer. 
Are tables purporting to tell what size picture lenses 
marked with a given focal length will project re- 
liable ? 158 

Projection Lens Diameter 

Is the diameter of the projection lens of vital import- 
ance to efficiency and good screen results? .147, 161, 184 

Under some conditions may a small diameter projec- 
tion lens cause light loss and unevenness of screen 
illumination? 147, 161, 184 

May a projection lens of too small diameter cause 
loss of "depth" in the picture? 147, 148 

Is a lens diameter in excess of that sufficient to ad- 
mit the entire light beam adventageous or the 
reverse ? 148 


Should the projectionist test his projection lens for 

distortion ? 148 

How mav a projection lens be tested for distortion? 

148, 149 


What must the projection lens do ? 149 

What two points are the conjugate foci points for 

the projection lens ? 127 

Explain why moving the projection lens nearer to 
or further from the film causes a change in "focus" 
at the screen. Fig 36D 149 



Altering Lenses 

Is it practicable to change the focal length of a 
compound lens (projection lens) by altering the 

distance between its front and back factors ? 150 

Matching Lenses 

Is it practical to order a lens to match the one you 
already have merely by giving the markings on its 
barrel? 140 

What must you send the manufacturer if you want 
a lens matched ? 140 

What precise measurements may you send with an 
order for a lens to match the one you have and 
expect fairly accurate, though not exact results ? . . 140 

How are matched lenses usually selected? 140 

What accurate data is sent by at least one manu- 
facturer along with lens shipments ? 140 

Repairing Lenses 

If one lens of a projection lens combination be 
broken or injured, may it* be replaced ? 139 

In ordering the replacement of a single lens element 
of a projection lens, what must be done ? \39 

Why is it necessary to send the broken element of 
a projection lens along with the unbroken ones 
when ordering replacement ? 139 

Have odd lenses or combinations of lenses any 
value? 139 

In case just one element of a lens combination be 
broken, how may the manufacturer ascertain the 
focal length of the broken lens ? 139 

Library, Projectionist's 

What books and publications may be recommended 
for the library of an up to date projectionist?.... 486 

License, Projectionist's 

General argument concerning projectionist license.. 478 

Is it good procedure, or a serious error to examine 
the competency of applicants for projectionist on 
electrics alone ? 479 

What danger has become increasingly important of 
late years? 479 

What total possibility of electric power loss is due 
to incompetent men in projection rooms? 480 

Is there value in competency in the individual pro- 
jectionist, even though he be too lazy to apply his 
knowledge ? 480 

Does licensing projectionists tend to increase effi- 
ciency? c 480 



Examining Boards 

Should examining boards be composed, wholly or in 
part, of men possessed of at least a fair knowledge 
of practical motion picture projection ? 481 

What should be the composition of a projectionist 
examining board ? 482 

Does the fact that a man may occupy high official 
position give any evidence that he is fit to be on 
an examining board ? 481 

What should be the general points covered by an 
examination ? 482 

License Law 

Give a rough draft of a competent license law 483 

Light Action 

(Also see "Optical Terms") 

1. What is meant by "Absorption of Light"? 19 

2. What is meant by "Actinic Ray" ? 19 

3. What is meant by "Angle of Incidence"? 

Fig. 65, page 222 

4. What is meant by the "Angle of Reflection"?... 222 

5. What is meant by the "Angle of Projection"?... 20 

6. What is a "Candle Foot," or "Foot Candle"?.... 23 

7. What is meant by "Standard Candle" ? 23 

8. What is a "Candle Meter" ? 23 

9. Quote the law relating to light intensity at dif- 
ferent distances from an open light source and 
explain its operation 125, 161 

10. Upon what is the law of light intensity, as per 
question No. 9, based ? 161 

11. What is the "Critical Angle"? 26 


Projection Room — (See "Projection Room") 
Chromatic Aberration 

What is "Chromatic Aberration" and what is its 
effect? 130 

What is meant by "Diverging Rays" or "Diverging 
Beam" ? 26 

What is meant by "Diffusion of Light"? 27,222 

What is meant by "Refraction of Light" ? 127 

What elements control the amount of bending light 
rays receive in passing from air to glass, or vice 
versa ? 127, 128 

For all practical purposes what may the projectionist 
assume the amount of refraction light rays will 
receive in passing through a lens will depend? 129 



Speed of Light 

What is the speed of light per second? 221 Y* 

Why will line FA, Fig. 27, page 125, be perpendicular 

to the surfaces of the lens ? 126, 131 

Why will the ray of light represented by line FA, 

Fig. 27, page 125, pass straight through the lens 

without refraction ? 126, 131 

Explain the refraction of the various light rays in 

Fig. 27, page 125. 
Explain the cause of refraction of light by a lens... 127 

Measurements and Calculations 

(Also see pages 481, 482) 

What two elements must be known to calculate the 
number of amperes flowing through any circuit?. 54 

In electrical calculations what do the letters E, C and 
R represent? 54 

In using common fractions what does the horizontal 
line mean ? 54 

How would you proceed to divide a smaller number 

by a larger one ? 54 


What does E/E or — mean ? 54 


What does E/R=C mean? 54 

E— 15 

What would mean ? 55 


What is the "Rule o' Thumb" for applying the for- 
mulas incident to the application of Ohm's law?.. 59 

What is the length, in fractions of an inch, of one 
mil? 70 

What is the area, in fractions of a square inch, of 
one square millimeter ? 395 

What is meant by the "Mil Foot Standard of Res- 
istance" ? 73 

How would you calculate the resistance of an elec- 
tric arc, amperage and voltage drop being known? 57 

How would you calculate the c p of an electric arc 
crater? 394 

How would you add or multiply, or divide or subtract 
common Actions ? 481, 482 

Circuit Voltage Drop 

How would you proceed to figure voltage drop in 
any given copper circuit ? 74 



How would you calculate increase in resistance 
caused by a known increase in temperature of the 
conductors ? 66 

How would you proceed to calculate the voltage drop 
of a circuit, its length and resistance per thousand 
feet of wire being known ? 69, 71 

How would you calculate the resistance, in ohms, of 
any copper circuit? 73 

Diameter of Spot 

How would you calculate the diameter of spot, diam- 
eter of light source, distance X and distance Y 
being known? 175 

Keystone Distortion 

How would you calculate the keystone distortion of 
a screen image, projection distance and projection 
angle being known ? 254 

How would you determine the amount of side view 
foreshortening due to angle of view, angle of view 
and viewing distance being known ? 252 

How would you calculate the added height of screen 
image due to projection angle distortion, projection 
angle and projection distance beng known? .. 254 

E. F. of Projection Lens 

How would you calculate E. F. of projection lens 
necessary to project a picture of given width, pro- 
jection distance being known ? 154 

How would you calculate size picture a lens would 
project at one distance, if size it projects at an- 
other distance is known ? 154 

How would you determine projection lens diameter 
necessary to admit entire light beam, free diameter 
of converging condenser lens and distance Y be- 
ing known ? Fig. 50, page 186 

Primary Amperage 

How would you calculate the primary intake am- 
perage of a motor generator, secondary amperage 
and voltage being known ? 116 

Light Source, Types 

(See Volume II) 

Mazda Projection 

887 to 924 (See Vol. II) 


Meters, Electric 

Describe : roughly, the action of an electric meter... 265 

How may you test your meter ? 265 

How would you read a meter ? 266 

Are all- meters to be read directly as per the dial 
showing ? 267 


How would you clean a projector mechanism after 

a film fire? 291 

How may Centegrade thermometer readings be re- 
duced to Fahrenheit?..., 512 

Centigrade and Fahrenheit Scales 481 

Melting Point of Metals 481 

Reflective Powers of various surfaces 481 

Common Fractions in calculations 481 

Newspaper and Fire Danger . A . . . 470 

Motor Generators and Mercury Arc Recti- 

(See "Current Rectification," page 493, Vol. II.) 

Optical Terms — Their Meaning 

What is meant by the "Principal Axis" of a lens?.. 126 
What does "perpendicular to" mean as applied to 

optics ? 126 

Explain the meaning of the term "Conjugate Foci". 127 
What is meant by "Refraction" as applies to optics? 127 

What is meant by "Angle of Incidence"? ... 129 

What is the "Angle of Refraction" ? 129 

What is meant by "Working Distance" ? 49 

What is meant by "Equivalent Focus" (E. F.) ? 129 

What is meant by "Spherical Aberration"? 129 

What is "Chromatic Aberration" ? 130 

What is meant by "Projection Angle"? 20, 255 

What is "Projection Distance" ? 38 

What is a "Condenser"? 25,164 

What is a "Light B-eam"?. 32 

What is a "Light Ray"? 32 

What is meant by "Working Distance"? 49 

What is the meaning of "Refractive Index"? 40 

What is meant bv "Regular Reflection"? 222 

What is meant bv "Diffuse Reflection"? 222 

What is meant by "Semi-Diffuse Reflection"?....-. 222 
What is meant by "Fade-Away"? See "Distribution 

of Light" 225 



Optical Train, Adjusting 

(See "Projection Optics, Practical") 

Optics, Practical Projection 

(See "Projection Optics, Practical") 

Picture, Distortion of The 

Explain, by diagram, why viewing a screen surface 
at an angle produces distortion of all objects 
thereon 252 

Will distortion of objects on screen increase as view- 
ing angle is increased ? 252-253 

Why do objects on a screen appear abnormally tall 
when viewed at an angle ? 253 

Keystone Effect 

When the lens is above screen center, what is the 
effect upon the screen image outlines ? 253 

Why does elevation of lens above screen center 
produce keystone effect in screen image ? 253 

How may the amount of keystone effect any given 
condition will produce be calculated ? 254 

Projection Angle Not Safe Guide 

Is projection angle a safe guide as to the amount 
of keystone effect which will be produced? . .255-256-257 

Real Evil of Distortion 

Wherein does the real evil of distortion lie ? 257 

How may the projectionist overcome keystone effect 

to the extent of making the sides of the picture 

parallel ? . 257-258 

Is kevstone effect accompanied bv out of focus 

effect ? 258 

How may depth of focus be increased and focus 

sharpened when keystone effect has injured it?.. 258 

Picture Size 

Are many things involved in "picture size"? 243 

General argument as to picture size 243-244 

What relation has picture size to distance screen 

to front seats ?..... 244 

Will the too-wide viewing angle caused by an im- 
proper relation of distance of front rows of seats 
from screen to picture size induce eye strain ? . . . . 244 
Will a too wide viewing angle caused by a picture 
too large for the distance screen to front seats 
tend to lower the value of the front rows of seats? 244 



What should be the relation of picture size to dis- 
tance screen to front seats ? 244 

What is the effect if the picture be too small for 
the distance screen to rear rows of seats ? 244 

What should be the probable maximum distance rear 
rows of seats to screen ? 244 

What may be considered as the minimum picture 
width for theatrical purposes ? 244 

What is one practical effect of picture size increase? 245 

What is the effect of picture size increase on light 
demand ? 246 

Does picture size increase make defects in the film 
more visible ? 246 

Ports, Projection Room 306 

Projection Room 

(See "Room, Projection") 

Projectors, Motion Picture 

625, Vol. II 

Projectors, Powers 

667, Vol. II 

Projectors, Simplex 

705, Vol. II 

Projectors, Motiograph 

739, Vol. II 

Projectors, Baird 

764, Vol. II 

Projection, Rear 

What is meant by "Rear Projection" ? 232 

How must a film be placed in the projector when 
rear projection (a translucent screen) is used? 232 

In rear projection do all titles on the screen read 
backward to the projectionist ? 232 

What is the chief reason why rear projection usually 
is not practicable ? 233 

What minimum focal length projection lens should 
control rear projection conditions ? 233 

Is it posible, using a theatrical size picture, too locate 
a translucent screen at the proscenium line, a pro- 
jector at rear of stage and get high grade results? 233 



Projection Room, The 

IMPORTANT NOTE— Under this heading you wi.l find many 
important things. Study it well. 


Why may the projection room rightly be styled the 
"heart" of the motion picture theatre? 293 

Will a wrong projection room location produce dis- 
tortion of everything in the screen image, includ- 
ing the picture outline ? 293 

While the slope of the picture sides may be remedied, 
will this remedy the distortion of the picture as a 
whole? 293 

Projection Angle 

What is the maximum projection angle as adopted 
by the Society of Motion Picture Engineers? 294 

How may you determine the height of lens above 
screen which will produce a 12 degree projection 
angle at any given distance ? 294 

How may the maximum height of projection room 
floor above screen center be determined to give a 
12 degree projection angle ? 294 

What is of first importance in considering projection 
room location ? * 294 

Projection Distance and Screen View 

How may you prove, in a simple way, that a too- 
great projection distance makes impossible a good 
view of details upon the screen ? 294 

If a good view of details of the picture is not had, 
will the projectionist be able to judge sharpness 
of focus closely ? 295 

Will an opera glass compensate perfectly for a pro- 
jection distance which makes it impossible to judge 
sharpness of focus with the eye ? 295 

Is it just plain common sense to presume that if the 
projection distance be such that the projectionist 
cannot have a sharp view of picture detail with the 
unaided eye, the picture will not be kept in sharp 
focus? 295 

Too Short Projection Distance 

Is a very short projection distance objectionable?... 296 
Front of Balcony 

Give us your views of the front of balcony projec- 
tion room location 296 

What points are for and against the main floor lo- 
cation ? 298 




What are the essentials of a first class projection 
room? ., 300 to 303 

How should the projection room door be made and 
hung? 303 

How should the floor of a projection room be sup- 
ported and made ? 304 

Is a perfectly solid floor essential to perfect pro- 
jection? 304 

May great damage be done to machinery and film 
by reason of an improper cement floor top dress- 
ing? 305 

Where the top of a cement floor slowly disintegrates, 
is it possible to apply a dressing which will cure 

the trouble? 305 

Wall Construction 

From what materials may the walls of a projection 

room be made ? 306 

Building in Conduit 

Should electrical conduit be built into the walls of 

projection rooms wherever possible ? 306 


What ports are necessary in a projection room? 306 

Do architects, as a rule, locate the various ports cor- 
rectly? 307 

How should the ports be located ? 307 

What should be the absolute minimum distance be- 
tween motion picture projector lens port centers? 307 

Should the distance betv/een motion picture pro- 
jector lens port centers exceed 36 inches unless the 
projection lens E. F. exceed six inches? , 307 

Should the center line of the screen, sidewise, fall 
midway between the motion picture projector lens 
ports? 308 

Minimum Dimensions 

What should the minimum dimensions of the obser- 
vation ports be, what should their height be from 
floor for level projection and how much should 
they be lowered for each five degrees projection 
pitch? .307, 308 

How may lens ports be filled in to area of opening 
just sufficient to pass the light beam?.^ 308, 309 

What should be the absolute minimum width for ob- 
servation ports ? • . 309 

How may a large observation port be made avail- 
able by means of a sliding shutter? 309, 310 



Glass Over Ports 

Is it practical to cover observation ports with glass? 310 

How should glass he set in an observation port? 311 

Is glass in lens ports permissible ? 311 

Does limiting size of ports by rule or law serve any 

good purpose ? 312 

How may the authorities expend their energies to 
better purpose than by setting up foolish limita- 
tions as to port sizes ? 312 

Spot Light Port 

What should be the approximate size and height of 

the spot light port ? 312 

Port Fire Shutters 

See Fire Hazard — Safety. 
Authorities in Error 

In what manner are authorities in error in the mat- 
ter of locating port shutter fuses ? 316 

Why should port fire shutter grooves be padded at 

the bottom? 316 

Projection Room Ventilation 

What three purposes does projection room ventila- 
tion serve ? 317 

How should the projection room vent flue be in- 
stalled? . 317 

Is an open vent flue safe or desirable ? 317 

What must govern the size of projection room vent 
flue, regardless of size of the room ? 317 

What should be the minimum area of vent flue of 
open type ? 318 

W r hat should be the minimum diameter of a vent flue 
in which a fan is used ? 318 

Is a 2-vent, 2-fan installation to be commended? 318 

Is it essential that metal vent flue be thoroughly in- 
sulated from all inflamable substance ? 318 

Is it essential to healthful conditions in the projec- 
tion room that fresh air intake ducts be installed? 318 

What state law is excellent regarding projection 
room fresh air inlet ducts ? 319 

Projection Room Equipment 

Read opening remarks under this heading carefully. 

319, 320 

Is it advisable that projection rooms be equipped 
with suitable closets for clothing, stores, spare 
parts, etc. ? 320 



If such conveniences be not provided, can the pro- 
jectionist be much blamed if things are not kept 

in order and there are delays ? 321 

Running Water and Toilet 

Why is a wash basin with running water and a toilet 
essential in a projection room ? 321 

Is it good practice to provide a chair or a stool at 

each projector ? 321 

Projection Room Reels 

Is it essential to good practice that special reels be 
provided to be used for projection ? * 322 

Projection Room Supplies 

Is scrimping on projection room supplies good policy? 327 

Is the projectionist who cannot be trusted to use 
true economy in the matter of projection supplies 
a fit man to have in the projection room at all?.. 327 

Is it true economy to try to get the last possible bit 
of wear out of projection room equipment?.., 327 

How may you tell whether or no the useful life of 
your asbestos covered lamp leads is finished, and 
replacement is true economy? 328 

Is there a tendency to use intermittent sprockets too 
long ? 328 

What proof can the projectionist offer that a too- 
economical policy in projection room supplies is 
not good business for the theatre management?... 329 

Does keeping projectors in use after three years of 
use represent good business management? 329 

Film Cabinet 

Name one high grade film storage cabinet 330 

The Rewinder 

Where should the rewinder be located? 332 

Is it of large importance that the elements of the 

rewinder be in perfect alignment with each other? 332 
Describe the proper arrangement of a rewinder and 

splicing block, etc 332, 333 

What should be the maximum speed of a rewinder?. 333 

Is rewinding film at high speed bad practice? 333 

Does "pulling down" in rewinding do great damage 

to film? 333 

Does a geared-down motor-driven rewinder relieve 
the projectionist of the task of watching the pro- 
cess of rewinding, unless inspection or repair is 

necessary ? 333 

Should there always be an ammeter and voltmeter 
in use ? 334 




Should the projectionist have a good tool kit? 335 

Suggest the necessary things in a projectionist's tool 
kit 335, 336 

Announcement Slides — See Slides. 

Projection Room Wiring 

Should the wiring of the projection room be care- 
fully planned before the room is built? 338 

How would you compute the size of wires necessary 
to supply the room? 338 

Suppose the projection room feed circuit be three- 
wire and the projection arcs must be connected to 
the outside wires, how must you then treat the 
circuit as to wire sizes ? 339 

Voltage Drop Waste 

Does an abnormal voltage drop in the projection 
room wires mean waste ? 339 

Suppose all current to be carried by the secondary 
of a motor generator, mercury arc rectifier or 
transformer located in or adjacent to the projec- 
tion room, what effect would that have on projec- 
tion room service w T ire sizes ? 339 

How should the projector circuits be run? 348 

Suppose the motor generator, mercury arc rectifier 
or transformer to be in the basement, what must 
the projection room feed circuit wire sizes be 
based upon ? 339 

May an acceptable projection room switchboard be 
built up from individual switches and fuse blocks? 

339, 341 
Voltmeter and Ammeter 

Should there be a voltmeter and ammeter in use in 
the projection room ? 334 

Where should the projection room ammeter be lo- 
cated? 334 

Where the projection room is supplied by a 3-wire 
circuit, should the power company be consulted as 
to method of connecting light sources thereto 341 

Should dissolver and Brinckert effects projector 
light sources always be connected to opposite sides 
of a 3-wire circuit ? 341 

Is there anything gained by connecting projection 
lamps to opposite sides of a 3-wire system if cur- 
rent be taken through rheostats? 88, 342, 343, 344 

Note — For Rheostatic Resistance see "Resistance as 
Applied to Projector Circuit." 



Are projector asbestos covered stranded lamp lens 
often used long after their replacement would have 
been true economy ? 327 

Connecting Motor Generators, Economizers — See those 
subjects under ''Current Rectification," Vol. II 

What should govern in locating the projection room 
switchboard ? 342 

How should projection room main fuses be placed?. 342 

Why do some power companies refuse to permit the 
neutral of a 3-wire system to be run to the pro- 
jection room? 342, 343 

Where the theatre is supplied by a 3-wire circuit 
and a motor generator, transformer or mercury arc 
rectifier is used to control projection light current, 
is the power company entirely within its rights in 
demanding that these devices be connected to the 
outside wires of the circuit ? 344 

Projection Room Lighting 

Is it optically an impossibility to have a clear, sharp 
view of the screen looking out of a well lighted 
room through a comparatively small opening? . .95, 344 

Is it probable that the best results will be had on the 
screen if the projection room be brilliantly lighted? 

95, 344 

Why s'hould the incandescent circuit switches be so 
located that the projectionist may reach them from 
working position beside the projectors? 95 

Should all projection room incandescent circuits be 
governed by one switch ? 95 

Are the effects of a brightly lighted room made 
worse if the observation ports be surrounded by 
a light colored wall ? 344 

Is it particularly essential that the projection room 
should be kept dark if the observation port is 
small? 345 

Describe one excellent, very efficient method of 
lighting the projection room 345 

How should the low-ceiling room be lighted? 345 

Has the theatre management the right to demand 
that the projection room be kept dark if darkness 
is essential to good screen results ? 345 

Projection Room Ground Wire 

Describe one method of installing a permanent 

ground for testing purposes 346 

Trouble Lamp 

How may a trouble lamp best be installed? •> 348 

Should all projection room switches be inclosed? ••• 348 



Double Throw Connection — See Switches. 

Polarity Changer — See Switches. 

Connecting to Two Sources of Supply — See Switches. 

Switching From One Set of Fuses to Another — See 


Grounds — See Grounds. 

Projection, Practical 

We earnestly recommend that you study this whole sub- 
ject fully and carefully. 

Do "Presentation" and "Projection" mean the same 
thing? 213 

What is the meaning of the term "Presentation"?... 213 

Upon what will the success of any production largely 
depend insofar as has to do with individual audi- 
ences ? 213 

Will or will not consistently high grade projection 
add materially to the "pulling power" of any thea- 
tre box office ? .213, 214 

Does or does not high class projection require ability, 
knowledge, ceaseless vigilance and artistic sense?. 214 

What range of knowledge is necessary to com- 
petency in motion picture projection ? 214 


Is it good business or even common sense for a 
theatre management to dictate the speed at which 
any given scene in a photoplay, or a photoplay as 

a whole shall be projected ? 215 

In what various ways is projection often hampered? 215 
Where a fixed time schedule for a show is employed, 
what is the and ONLY proper course to pursue. . . . 2'16 

Over- Speeding 

Is over-speeding a photoplay good practice? 216 

May the speed of projection necessary for best re- 
sults vary even in adjoining scenes of a photoplay? 217 

W T hat is the correct projection speed for any scene 
or subject ? 217 

What is one of the highest functions of the pro- 
jectionist? 217 

Is the man who regards the finer details of projec- 
tion as of slight importance to a high grade pro- 
jectionist ? 217 



Make Good Work Impossible 

Do theatre managers and exhibitors often impose 
conditions which make high class projection an 
impossibility ? 217 

Will the theatre management which pays for high 
grade projection, grants conditions which makes it 
possible, and then insists upon its delivery, be sure 
to finally reap reward at the box office ? 218 

Over -Speeding Inexcusable 

Is there any possible legitimate excuse for over- 

. speeding projection? 218 

Film Reel Construction — See Reels. 

Leader and Tail Piece 

Why is it essential that there be a leader and a tail 
piece on every reel of film ? 284 

What should be the minimum length of leader and 
tail piece ? 284 

Why is a leader and tail piece necessary on each 
reel of a multiple reel production ? 285 

Why should the tail piece be opaque ? 285 

Threading Out of Frame 

Is it ever permissible to thread out of frame? 284 

How would you clean a projector mechanism after 

a film fire ? 291 

White Light on Screen 

Should the white light be permitted to show on the 

screen at the end of a reel ? 286 

The Rotating Shutter 

(Also see pages 644 to 658, Vol. II.) 
Is the rotating shutter in effect a part of the optical 

system of a motion picture projector ? 206 

Is the shape of the beam of light emerging from 

projection lenses always the same shape? 206 

Aerial Image 

What is the "Aerial Image"? 205 

Shutter Position 

What is the correct position of the rotating shutter? 206 
How may you ascertain the position of the aerial 

image ? 206, 207 

Is there any advantage in placing the rotating shut- 
ter at the point of least diameter of the light beam, 
usually the aerial image, unless something else be 
done ? 207 



Is the diameter of the light heam sometimes less at 
a point nearer the lens than the aerial image?. 207, 208 

If the beam be parallel for a distance in front of the 
projection lens is there advantage in locating the 
shutter at the aerial image? 207 

What effect has increasing projection lens diameter 
upon the rotating shutter? 208 

Lens Diameter and Blade Width 

Why is it true that even though a small diameter 
lens admit the entire light beam, if a larger diam- 
eter lens be substituted it will be found necessary 
to increase the width of the master blade of the 
rotating shutter ? 208 

Light Loss Caused by Rotating Shutter 

Approximately what percentage of the total light 
does the rotating shutter cut off? 208 

Is there advantage in placing the rotating shut- 
the light beam as far as possible from the center 
of the shutter shaft ? 208 

Why ought the projectionist to study his local con- 
ditions closely particularly with relation to the rot- 
ating shutter ? 209 

Is flicker due to action of rotating shutter ever jus- 
tified? 235 

Projection Optics, Practical 

Is it difficult to select the condenser and the projec- 
tion lens and so adjust them that they will work 
together at maximum efficiency? 160 

Describe the function of condenser and projection 
lens 126, 160 

Is it possible under all conditions to cause the con- 
denser and the projection lens to work together 
with high degree of efficiency 160 

May a projector optical train be judged, as to its 
action, by ordinary optical standards? 160 


What elements of inefficiency may be set up as be- 
tween the condenser and projection lens if the pro- 
jectionist lacks intimate, expert knowledge of them 
and their action ? 160, 161 

What is the result if the projection lens passes to 
the screen only a portion of the light passing 
through the aperture ? 161 



What various difficulties are there in connection 
with the projection lens ? 161 

What objection is there to a large diameter projec- 
tion lens ? 161 

Large Lens Diameter Objectionable 

Under what conditions would a large diameter pro- 
jection lens be highly objectionable ? 161 

How may we demonstrate the correctness of the law 
which tells us that light intensity varies inversely 
as the square of the distance ? 161 


What diameter collector condensing lens located 4J/2 
inches from a light source would be necessary to 
collect the same amount of light that a 4^4-inch 
diameter lens would collect at 2% inches from the 
same light source ? Fig. 36H, page 162 

Unless refracted, how do light rays travel after leav- 
ing the light source ? 162 

Does not Fig. 36H, page 162, prove to you the great 
importance of keeping the light source as close as 
practicable to the collector lens ? 162, 163 

Are the results indicated by Fig. 36H, page 162, sub- 
ject to some modification? And if so, why? 163 

What percentage of the total light is concentrated 
upon 50 per cent of the collector lens area? 163 

Of Basic Importance 

What is the first important point in the adjustment 
of the projector optical train ? 163 

What one factor forms the basis of all correctly made 
lens charts ? 163 

How is the light source located with relation to the 
collector lens when the Griffith Lens Chart is 
used ? 164 

Optics of Condenser — See "The Condenser" 

Location of Crater Image 

Where is the crater image located in stereopticon 
projection ? 172 

Is it impracticable to focus the crater in the projec- 
tion lens in motion picture projection? 172 

In practice where has it been found best to locate 
the crater image in motion picture projection? 172 

Why cannot the crater image be focused too far on 
the projection lens side of the aperture ? 172 



The Spot 

When using the ordinary arc, is the shape of the spot 
a direct indication of the condition of the crater?.. 174 

What does a round spot indicate when using the 
ordinary arc ? 174 

What is the minimum size of spot it is practical to 
carry ? 1 74 

Why should the spot not be too small ? 174 

Light Loss at Spot 

What is the difference in light loss with a 1.5 and 
a spot 2.25 inches in diameter ? 173 

What is the obvious lesson conveved by Fig. 41, page 
173? 174 

When using ordinary or high intensity arcs, what 
is the main thing to consider in selecting the focal 
length of the condenser ? 175 

Under v/hat condition is a long Y distance (distance 
face of converging lens to aperture) of import- 
ance? 175 

What relation has distance center of condenser to 
light source and to aperture to size of spot? 175 

Computing Spot Diameter 

What is the rule for computing diameter of spot 
when diameter of light source, distance apex of 
curved surface of collector lens from light source, 
distance apex of curved surface of converging lens 
to cooling plate are known and spacing between 
lenses of 1/16 of an inch ? 175 

Is the projectionist who has a correst condition in- 
crease his amperage and reduce spot by pulling the 
light source back working intelligently, or how 
should he (see previous question) have reduced 
the spot diameter ? 176 


What is the real problem in relation to condenser 
focal length? 176 

Slide Carrier Waste 

If a slide carrier is permitted to remain in front of 
a 4^4-inch free opening condenser during motion 
picture projection, what percentage of the light is 
cutoff? 177 

Ghost Zone in Condenser Beam 

Why is there a well defined ghost zone in the beam 
projected forward by a plano-convex condenser?. 177 



How may we demonstrate the chromatic aberration 
produced by the plano-convex condenser? 178 

Explain why white light is had at the spot when a 
plano-convex condenser produces chromatic aber- 
ration 178, 179 

Does focusing the arc crater at the film plane tend 
to produce white light ? 179 

Adjusting Optical Train of Projector 

What should be done before attempting the adjust- 
ment of the projector optical train ? 179 

In adjusting the projector optical train in conformity 
with lens tables, why must it be done with under- 
standing and intelligent care ? 180 

Does Figs. 46 and 47, together with the text on pages 
181 to 183, convince you that even screen illumina- 
tion is impossible unless the projection lens admits 
the entire light beam from the aperture? 

Light Beam Diverges Beyond Aperture 

Examine Fig. 48 and text, page 184, for evidence of 
diverging light beam 184 

Examine Figs. 46, 47 and 48 and analyze them care- 
fully 181, 182, 184 

Projection Lens Diameter and Condenser Distance 

Examine Fig. 50, page 186, and text page 185, and 
Fig. 51, page 187, for evidence of effect of con- 
denser distance (distance Y) on divergence of light 
beam beyond aperture 186 

Is it comparatively simple to secure evenness of 
screen illumination? 181 

Light Loss Through Divergence of Beam 

Figs. 52, 53 and 54, together with text pages 187 to 
192, sets forth the enormous loss of light made 
possible by divergence of the beam beyond the 
projector aperture under unintelligent handling. 

Lens Chart 

Should every theatre and projectionist have a Grif- 
fith Lens Chart? 192 

Is it possible to secure the best results on the screen 
unless there be careful, scientifically correct adjust- 
ment of correctly selected elements of the pro- 
jector optical train ? 193 

May incorrect selection or adjustment of the various 
elements of the projector optical train produce 
very bad, inefficient results ? 193 



How may the projectionist ascertain for himself the 
condenser focal length and distance Y (lace 01 
converging lens to aperture) with relation to pro- 
jection lens diameter? i> 


What caution should be exercised in projection lens 
diameter and distance Y ? 195 

If the projection lens be say 2.25 inches in diameter, 
and a Y distance of 18 or 19 inches will not get 
the entire beam into it, what should be done? 195 

If the projection lens in use be of maximum diameter 
and cannot be made to admit the entire light beam, 
what would you do ? 195 

Is there a universal method of selecting and adjust- 
ing the various elements of the projector optical 
train? ;•••.••-. 198 

Is it always possible for the projectionist to secure 
the diameter of projection required for his needs? 198 

Is it important to have the correct optical lineup 
otherwise, even though the correct diameter pro- 
jection lens cannot be had ? 198 


Can the best results be had on the screen unless the 
various elements of the projector optical tram be 
intelligently selected and carefully and correctly 
adjusted with relation to each other? 202, 203 

Why is there tendency to chromatic aberration in the 
outer zones of the condenser lens ? 203 

Why does the color in the outer edges of the con- 
denser beam reach its center at the spot? 203 

Is it possible that a condenser of smaller diameter 
than the present standard be more efficient? 203 

Whole Condenser Cannot Be Used 

Is it, under some conditions, imposible to use the 
entire diameter of the condenser ? 204 

Shape of Beam 

Does the light beam always come from the projec- 
tion lens in the form of a parallel or a diverging 
beam? 206 

Will the shape of the beam as it emerges from the 
projection lens be dependent upon the projection 
distance and width of picture ? 206 

May the light beam come from a long focal length 
projection lens as a parallel or slightly converging 
beam? 206 



At what point in front of the projection lens is there 
a dissolving effect on the screen when an opaque 
object is passed through the light beam? 207 

For things directly connected with the projector 
rotating shutter see "ROTATING SHUTTER." 

Eye Strain 

Why do many people avoid the motion picture the- 
atre ? 234 

Why does "flicker" produce or set up heavy eye 
strain ? 234, 235 

Reason for Flicker 

To what is flicker due, and is it always possible for 

the projectionist to eliminate it ? 235 

Does the screen ever produce flicker? 235 

May a change of screen surfaces influence flicker 

tendency ? 235 

Is flicker due to action of rotating shutter ever 

justified? 235 

Does lack of sharp definition (sharp focus) set up 

eye strain ? 235 

What simple proof may be offered that "fuzzy focus" 

causes eye strain ? 235 

To what may poor definition be due ? 235, 236 

Screen Illumination and Eye Strain 

Why does insufficient screen illumination induce eye 
strain ? 236 

Draw an analogy between reading a book by poor 
light and watching a shaky, jumpy or poorly illum- 
inated motion picture . . , 236 

Has size of picture an effect upon eye strain? 236 

Why does nearness of observer from large picture 
involve eye strain ? 237 

Glare Spots and Eye Strain 

Do glare spots induce eye strain ? 237 

Is it possible for ever-illuminated highlights in the 
picture to become, in effect, glare spots? 237, 238 

What should be the maximum permissible illumina- 
tion of any and all things, except the screen, vis- 
ible to the motion picture theatre audience? . .237, 238 

Name some of the glare spots ordinarily found in 
theatres 238 

What is the apparent brightness of a sheet of white 
paper (music) illuminated by a 25 watt incan- 
descent lamp at one foot ? 238 



Fixing the Clock 

How is it practicable to make the telling of time 
from a clock very easy, without setting up a glare 

spot? 238 

Exit Lights 

How may exit lights be made very conspicuous with- 
out causing them to be glare spots ? 239 

Merely because no one makes complaint of glare 
spots is that proof that there are none in your 

theatre? 239 

Definition and Picture Magnification 

Is sharpness of definition decreased by picture mag- 
nification ? 245 

Picture Size and Light Demand 

As the picture size or area is increased is it neces- 
sary to increase the light if screen illumination is 
to remain constant ? 245 


How many times is the film photograph magnified in 
ordinary picture sizes ? 245 

How many film photographs would it require to 
cover the area of a 16-foot undistorted picture?.. 246 

What is the relative area of different width undis- 
torted pictures, and what would the relative brilli- 
ancy of each be, if light incident thereon remain 
constant? Table 10, page 245^ 

Magnification of Defects 

What importance has the defects in old film which 
bears directly upon picture size ? 245y 2 

How wide w r ould a scratch mark l/64th of an inch 
wide in the film appear in a 12-foot picture? How 
wide in a 20-foot one ? 246 

Would side or other movement in film be more vis- 
ible in a large picture than in a smaller one? 246 

Explain, by diagram, why viewing a screen at an 
angle sidewise produces distortion of objects 
thereon 252 

Projector, The Motion Picture 

(For detailed instruction on various makes of motion pic- 
ture projectors see pages 625 to 666; for High Intensity 
Arcs see pages 786 to 803; for Reflector Type Arc Equip- 
ment see pages 817 to 825, and for Mazda lamp instruction 
see pages 887 to 924, all in Vol. II.) 



The Lamphouse 

Is a roomy, well ventilated lamphouse essential to 

high class work ? 362 

What changes in lamphouse construction has time 

wrought ? 362 

Is lamphouse ventilation of great importance ? 362 

What harmful effects may lack of ample lamphouse 

ventilation have ? 362 

Of what is the grey-colored ash which deposits on 

the lamphouse interior formed? 363 

Lamphouse Ventilation 

What is the best method of lamphouse ventilation?. 363 
Is the vent pipe shown in Fig. 104 essential to 

healthful conditions in the projection room? 364 

Can a vent pipe such as is shown be installed on a 
lamphouse which must shove sidewise to accom- 
modate stereopticon projection ? 364 

Crater Image Projector 

Should a crater image projector be attached to the 
lamphouse ? 364 

Describe one method of automatically lighting in- 
terior of lamphouse when door is opened... 365 

Are asbestos covered lamp leads usually kept in use 
too long? 327 

Should a condenser holder have a means for altering 
the distance between the individual lens holders?. 365 

Why do condenser lenses break ? 365 

What are the various requirements of a condenser 
holder? 367 

Is it best to locate the condenser inside or outside 
the lamphouse ? 368 

Is an inside dowser which comes between the light 
source and condenser to be commended? 368 

Clean Lamphouse 

Is it essential that the lamphouse be kept clean ? 368 

Should the lower carbon jaw strike the metal of the 
lamphouse when the lamp is advanced, how may 
a ground be prevented ? 369 

Ordinary Straight Arc Lamp 

The Lamp 

Is it possible to secure good screen illumination 

with a lamp in poor condition ? 369 

What adjustments should a projector arc lamp have? 370 
What kind of insulation is used for arc lamps? 370 



What should the projectionist be very careful about 
with relation to lamp insulation? 370 

Should the projectionist be careful to keep the lamp 
free from carbon dust ? 370 

If carbon jaws do not make perfect electrical con- 
tact with the carbons what will be the result? 371 

Does poor contact between carbon and carbon jaws 
induce penciling of the carbon? 371 

Name one efficient, easily applied method of clean- 
ing the contact surface of carbon jaws 371 

Lamp Lubrication 

Is it important that the moving parts of the lamps 
be lubricated at stated intervals? 371 

How should lamp parts subject to heat be lubri- 
cated? 371 

Is proper lubrication of the carbon clamp screws of 
especial importance ? 371 

Asbestos Lamp Leads 

Will there be heavy loss by reason of high resistance 
unless lamp leads be handled intelligently? 372 

Installing Circuits 

How should the circuits which supply the projector 

light sources be run ? 348 

Is it essential that the projector, as a whole, be firmly 

anchored ? 334 

Is it good business policy to keep a motion picture 

projector in service more than three years? 329 

Is it good business policy to permit projectors to 

get into a state of poor repair ? 329 

Is there a tendency to use intermittent sorockets 

too long? * 328 

How could you clean your projector mechanism after 

a film fire? , 291 

Projectionist, The 

What is one of the highest functions of the projec- 
tionist ? 217 

What, is one of the first things a projectionist should 
do when taking a new position ? 68 

Is it necessary to efficient work that the projec- 
tionist be able to calculate the resistance of cop- 
per circuits, calculate voltage drop, etc.? 68 

Is it the part of wisdom for a theatre manager to 
employ a projectionist in whose ability he has not 
at least a reasonable amount of confidence? 328 



What records of supplies should the projectionist be 
required to keep? 328, 329 

What proof may the projectionist offer that a too- 
economical policy in projection room supplies does 
not pay? 328 

What tools should the projectionist himself own? 

335, 336 

Should the projectionist have a good tool kit and 
the tools be kept in perfect order ? 337 

Does poor tools or tools in poor order mean vexa- 
tious delays? 337 

Is the one who regards the finer details of projec- 
tion as of slight importance a high grade projec- 
tionist? 217 

Rectifiers, Mercury Arc 

567 to 595, Vol. II 

Reflector Arc Lamp 

817 to 834, Vol. II 

Report, Projectionists' 

What form of report should be made by the projec- 
tionist concerning films received from an ex- 
change ? 466 

What should be done with these reports ? 466 

Would such reports have a beneficial effect? ,. . 466 

In what condition has the theatre a moral and 

LEGAL right to expect to receive its films? 466 

Do films in bad mechanical condition set up a dan- 
gerous condition ? 467 


Is there any publication which gives all details con- 
cerning all power plants located in the United 
States ? 85 


Does flimsy reel construction work great damage 
to film? ....281, 322 

What harmful effects do small diameter reel hubs 
have? 281 

Is the "saving" accomplished by light, flimsy reels 
far more than offset by the damage they do to 
film? .... 281 

Should the sides of reels be strong and rigid? 281 



Does the exhibitor eventually have to pay for all 
damage done to film, regardless of how or where 

it was done ? 282 

Is it advisable to have 1,000, 2,000 or 3,000 foot reels? 282 

Should reels ever be completely filled with film? 282 

What are the various evils of the overloaded reel?. 282 
How may one calculate the footage of film on a 
reel? 283 

Projection Room Reels 

(Also see page 640, Vol. II.) 

Should special reels be provided for use in the 
process of projection ? 322 

Name two high grade reels suitable for use in pro- 
jection rooms 323, 324 


(Also see "Wire Systems") 

What do you understand resistance, as applies to 
electricity, to be and what is its cause ? 53 

With a wire of given diameter, what various things 
will tend to increase or decrease its resistance? .53, 60 

When the voltage and amperage is known, how is 
the resistance calculated ? 55 

Quote the substance of Ohm's law 55 

When is the normal capacity cf a water pipe or an 
electric conductor said to have been reached? 62 

What is the "Temperature co-efficient"? What does 
it represent ? 66 

Is the resistance of an electric conductor to all in- 
tents and purposes constant until its normal am- 
perage capacity is reached ? 71 

Is it practicable to calculate the resistance any con- 
ductor of given length will offer at any given 

temperature ? 66 


(See "Resistance, as Applied to the Projection Circuit.") 
Calculating Resistance 

By what means may the projectionist quickly ascer- 
tain the resistance of the wires of his circuits? 
Table No. 2, page 72 

Is it practicable for the projectionist to apply knowl- 
edge of voltage drop due to resistance in a prac- 
tical way in his work ? 69 

How does increasing or decreasing the diameter or 
length of a water pipe or electric conductor affect 
its resistance, assuming water or current flow to 
be constant? •. 63, 64 



What is the practical effect of resistance — in what 
form does it manifest itself ? 64, 65, 68 

What are the various factors to be considered in 
selecting the metals for electric conductors? 67 

How would you calculate the resistance of a Mazda 
lamp filament? 55 

Resistance as Applied to the Projection Cir- 

Why is it that resistance as applied to the projector 
circuit should have separate, extended considera- 
tion ? 413 

What should be done before taking up the study of 
resistance as applied to the projector circuit? .413, 414 

Of what does a "rheostat" consist ? 414 

What Rheostat Does 

Exactly what does the rheostat do? . 414 and 428 

Upon what does the number of amperes a rheostat 

will permit to pass depend ? 414 

What effect does the arc itself have upon the total 

resistance of a rheostat controlled arc circuit? 414 

Total Resistance 

Upon what total resistance will the amperage at the 
arc depend ? 414 

How may the current flow be altered in a rheostat- 
controlled projector arc circuit ? 415 

Variable and Fixed Rheostats 

Explain the operation of a variable and fixed resist- 
ance rheostat 415 

Are man)' rheostats so made that their resistance 
may be altered merely hy moving a lever or mani- 
pulating a switch ? 416, 436 

What is the least amount of fixed resistance an ad- 
justable rheostat may have ? 417 

Maximum Load 

What difference is there between a rheostat coil and 
grid? 417 

Should a rheostat be so loaded that its coils or 
grids appear red ? 417 

What ought to be the maximum operating tempera- 
ture of your rheostat grids or coils ? 418 

Rheostat Repairs 

Should an extra coil or grid to fit the rheostat be 
kept in . stock ? 418 



What is the main point to remember in making 
rheostat repairs ? 418 

What path does the current follow in passing 
through the rheostat ? 418 

What is consumed as the current flows through the 
rheostat? 418, 419 

Why does the arc voltage not vary as rheostatic 

resistance is varied ? 419 

Insulating Rheostats 

Why should the rheostats be insulated, as a whole, 
from earth ? 419 

Is heavy current loss often traceable to rheostats not 

properly insulated from earth ? 419, 420 

Cooling Rheostats 

How may rheostats be kept comparatively cool? 420 

Location of Rheostats 

What precautions must be taken with regard to 
proximity of rheostats to inflammable material?.. 420 

Where is it best to locate the rheostats? 420 

Is it practicable to locate adjustable rheostats out- 
side the projection room and still handle the ad- 
justment from projection position ? 420, 421 

If rheostats are located in the projection room, 

where ought they to be placed ? 421 

Examining Connections 

Should rheostat connections be examined and thor- 
oughly cleaned at stated intervals ? 421 

How may added resistance be installed if your 
rheostat passed too much current ? 421 

What objection is there to a rheostat made of or- 
dinarily iron wire ? 422 

Big Grid Rheostats 

What is the objection to large grid rheostats? 422 

Is it possible to effect temporary repair if a coil or 

grid burns out ? 423 

Rheostat Connections 

What is meant by a "series" connection of rheostat? 

Fig. 134, page 424 

What is meant by a "mutiple" (same as parallel) 
connection of rheostats ? . , Fig. 136, page 425 

What is the effect of the series and mulltiple rheo- 
stat connection ? 423 to 427 

Is there any such thing as an A. C. or D. C. rheo- 
stat ? 427 

Is there any difference in effect when using a rheo- 
stat to control a D. C. and an A.C. projection arc? 427 



Rheostats Wasteful 

What is the purpose of a rheostat as applies to the 
projection circuit ? 428 

How is the energy absorbed in the rheostat dissi- 
pated? .•••:••••; 428 

As applies to the projection circuit is rheostat waste 
increased as voltage is increased ? 428 

To what projection circuits should the use of 
rheostats be restricted ? 42 ( > 

Should a rheostat be used where A. C. is used at 
the arc? 42^ 

May a coil and grid rheostat be used together? 

Fig. 138, page 430 

May a fixed resistance and a variable resistance 
rheostat be used together ? 429, 430 

Does the resistance of a wire coil rheostat increase 
with use ? 432 

How are rheostat coils attached to but insulated 
from the frame ? 433 

Which is the better for road use, the grid or wire 
coil rheostat ? 436 

The Multiple Coil Rheostat 

Describe the multiple coil rheostat and its electrical 
action Pages 437 to 441 

Rewinding .270 and 332 

Screen, The 

(Also see "Characteristics of Screen Surfaces, Page 
483 to 492F, Volume II. 
What is the sole and only function of a theatre 

screen ? 219 

Name one fundamental requirement of a good screen 

surface 225 

What effect upon the viewing of a picture has 

abundant projection light? 219 

What is the effect of a too-brilliant screen on the 

eye ? 219 

Why does light shining upon the screen other than 

light from the projection lens tend to injure the 

contrast in the picture and make it appear dull 

and gray ? 219 to 225 

Light Tone 

Why should the light reflected from the screen have 
a proper tone or color ? 219 



A Great Difference 

Is there a great difference in results with equal light 
intensity but different screen surfaces ? 219 

Can the relative merits of screens in different thea- 
tres be judged by watching their performance 
separately? 219, 220 


Explain why it is impossible to judge of the relative 
merit of screens in different theatres merely by 
watching their performance 220 

Name the various things which may affect the results 
as viewed on any screen, aside from screen sur- 
face itself 220 

Is it even possible to judge closely of two screen 
surfaces by substituting one for the other while a 
picture is being projected ? 220 

How may two screen surfaces be fairly compared 
as to their performance ? 220 

Is it wise to make hasty judgment between screen 
surfaces? 220 

Salesmen's Statements 

Should the statements of screen salesmen be ac- 
cepted without supporting evidence ? 221 

What happens to the eye when we attempt to view 
a picture on a highly illuminated screen surface 
of a specular nature ? 221 

How may you prove to yourself that a specular 
(highly reflective within a narrow angle) screen 
highly illuminated causes loss in detail of shadows? 221 

Can You Answer These Questions? 

What is of large importance with regard to the 
screen surface in theatres where the rear seats 
are not an excessive distance from the screen.. 221 J/2 

Is it of importance that the projectionist study 
screen surfaces ? 221H 

What should exhibitors always do before purchas- 
ing a screen ? 221^4 

If neither the exhibitor or projectionist cares to ap- 
ply the tables and charts herein provided before 
purchasing a screen, what should be done 22\ l / 2 

Types of Reflection 

What is meant by "diffuse," "semi-diffuse" and "reg- 
ular" reflection ? 222 

What is the effect of diffuse reflection as applies to 
the theatre auditorium ? 222, 223 



Upon what element in the screen surface does the 
amount of diffusion depend ? 223 

May a screen surface have both the elements which 
give regular reflection and diffuse reflection? 223 

in case a sort of haze appears before the screen, 
what is the cause ? 223 

Has a screen surface of visible roughness any ad- 
vantage as against a plain unpolished surface, such 
as plaster ? 223 

Interfering Light 

What is meant by "interfering light" ? 224 

Is interfering light a serious matter ? 224 

Name some of the possible causes of interfering light 224 
Should projectionists make frequent tests for inter- 
fering light? ;. . . 224 

How would you test your screen for interfering 

light? 224 

What should be done after testing the screen for 

interfering light with the entrance doors closed ? . . 224 
Can a screen which receives interfering light give 

the best results ? 225 

How may windows be made light-tight ? 225 

Standing at the screen, looking toward the seating 
space, should any unshaded light source be visible 

to the eye ? 225 

What is meant by "fade-away" ? 225 

To what type of theatre auditorium should a screen 
surface having heavy fade-away be restricted?... 226 

Types of Screen Surface 

Into how many general classes may screen surfaces 
be divided ? 226 

White Plaster Wall 

Name the various reasons why a white plaster wall 

is a good screen surface 226 

How may a plaster surface screen be cleaned? 226 

The Cloth Screen 

What are the relative powers of diffusion and reflec- 
tion of the cloth screen ? 226 

What kind of cloth is best for a screen? In this we 
refer to just ordinary plain, unprepared cloth 226 

How wide may cloth be had ? 226 

How should a cloth screen be stretched? 227 



The Painted Surface 

What element in a paint determines its p >vver to 
reflect light ? 227 

How may a paint be produced which will have very 
high light reflection powers ? 2ZJ 

How much higher reflection power may a paint be 
made to have by careful compounding, as com- 
pared to ordinary paint of the same color? 227 

How fast do ordinary house paints deteriorate be- 
cause of chemical changes in the paint itself?.... 227 

Do water color paints (kalsomines) deteriorate in 
light reflection powers in the same proportion as 
paints, and for the same reason ? 227 

Why should black never be used to "whiten" screen 
paint ? 227 

What foundations may be used for a painted screen 
surface ? 227 

What home-mixed paint is as good as any screen 
paint, and much better than many? 227 

Is the addition of any color to a screen paint ad- 
visable? i 227, 228 

How many coats of paint should be applied to a 
screen ? 228 

What advantages have painted screen surfaces?.... 228 

Is a painted screen surface best for use in certain 
types of theatre auditorium ? 228 

Has paint on the rear surface of a screen any real 
value in increasing the reflection power of the 
screen surface ? 228 

What should be done to a plaster surface before 
coating it with paint ? 229 

What will be the effect of washing a painted screen 
surface ? 229 

What is the reflection powers of a painted screen as 
compared to magnesium carbonate ? 

Kalsomine Screen Surface 

Is kalsomine a good screen surface ? 228 

What is the relative powers of reflection and dif- 
fusion of kalsomine as compared with other screen 

surfaces ? Table 00, 

How should kalsomine be applied? 229 

How often should a kalsomine surface be renewed 

for the best possible results ? 229 

Because the surface of a kalsomine (or any other 
kind for that matter) screen LOOKS clean, is that 
proof that it is ? 230 



How may you test your kalsomine or painted screen 
surface ? 230 

May "neat" (white) cement be used for a screen 
surface ? 230 

What should be done to cloth before it is either 
kalsomined or painted ? 230 

Metallic Surface Screens 

What is the sole advantage in the metallic surface 
screen ? 230 

Of what is the surface of a "metallic surface" screen 
mostly composed ? 230 

Over what maximum angle does the relatively high 
brilliancy obtained by using a metallic surface 
screen extend, and what happens beyond that 
angle — also, to an extent, within it?..., 230 

Is there tendency to discoloration of a metallic sur- 
face screen ? 230 

What guarantee should every purchaser of a metallic 
surface demand ? 231 

Where may the characteristics of different metallic 
surfaces be examined ? 232 

Should the attempt to make "home-made" metallic 
surface screens be avoided ? 231 

Chalk Surface Screen 

How may a chalk surface screen be made and its 
surface renewed ? 231 

Does a well made chalk surface give good picture 

results? 231 

Mirror Screens 

Of what does a mirror screen consist ? 231 

To what element inherent in the mirror itself is the 
out-of-focus effect when viewed at wide angle 
due ? 231, 232 

What type mirror screen is ideal for a long, narrow 
auditorium ? 232 

Name one principal objection to mirror screens.... 232 

Name one advantage of the mirror screen 232 

Translucent Screens 

Of what is a translucent screen made ? 232 

If cloth be used for a translucent screen, how must 

the lens be located ? 232 

Why is it impracticable to use a cloth screen and 

rear projection in a theatre having a balcony? 232 

If a cloth screen be used for rear projection, is the 

result improved if the cloth be kept wet? 232 

What is the best translucent screen ? 233 



What is the principal objection to using tracing 
linen for translucent screens ? 233 

What other objection is found to rear projection, 
based on the necessary screen position? 233 

Is it possible to place a translucent screen at the 
proscenium line, set the projection at rear of stage 
and get a good picture ? , , , 233 

The Concave Screen Surface 

Is there advantage in using a screen having a con- 
cave surface ? 233 

Height of Screen Above Floor 

What should be the height of the screen above the 
floor? 234 

Eye Strain — See pages 224 to 239 and page 244. 

Flat Surface 

Should the screen surface be perfectly flat and set 
as nearly as possible at right angles to the axis 
of projection ? 239, 240 

Outlining the Picture 

Should the picture be outlined ? 240 

Why should the picture border extend somewhat on 

or into the picture itself ? 240 

What is the effect of outlining or bordering the 

picture ? 241 

Has an unbordered picture the same sharp, pleasing 

contrast a properly bordered one presents? 241 

Will any reasonably dark non-gloss color be satis- 
factory for an outlining border ? 241 

Do some high authorities disapprove of the use of 

black for a picture border? 241^ 

How may the materials and colors best suited to use 

in any given case be tested for selection? 242 

What booklet is excellent for consultation in theatre 

illumination and screen selection ?,,,,..., 242 

Screen Surroundings 

Should the immediate screen suroundings be non- 
gloss ? 243 

What injury may be done to the picture where a 
large orchestra is employed, and the screen sur- 
roundings of sort which will reflect light? 243 

Should the stage floor be covered with a dark, non- 
reflecting paint or material under some conditions? 243 



Size of Screen (Picture) 

Why does the small picture often look abnormally 
small? 244 

What will be the effect upon patrons seated in front 
rows if they be near the screen and the picture 
very large ? 244 

Has the picture size a direct bearing on the value 
of the front rows of seats in any theatre? 244 

Will a large picture cause eye strain for those seated 
near to it ? 244 

What should be the minimum distance of a 16 foot 
picture from the front row of seats ? 244 

What must be considered in connection with size of 
picture and distance rear seats to screen? 244 

What distance may a well illuminated 16 foot picture 
be from the rear row of seats without causing eye 
strain to patrons seated in those seats? 244 

Screen Tinting 

Is it possible to tint the surface of a screen with 
beneficial effect ? 246 

If a screen surface is tinted, how must it be done? . . 246 

What guide as to screen tinting does one manu- 
facture practice ? 246 

Are the possibilities yet unexplored as to tinting the 
arc light at its source ? 247 

Screen Location 

Has experience proven that locating the screen at 
end of auditorium where audience enters is not 
good practice ? 247 

Is screen location at entrance end of auditorium de- 
sirable from the fire hazard viewpoint? 247 

What should the authorities give their attention to 
in the matter of making the theatre safe as to fire 
hazard? 247 

Where screen is located on stage of straight motion 
picture theatre, what should be minimum distance 
front row of seats from screen ? 247, 248 

In a shallow theatre why should the screen be placed 
as far back on stage (if there be one) as possible? 248 

Do many vaudeville-motion picture theatres show in- 
difference to the proper presentation of the pic- 
ture? 248 

How should the screen be mounted in theatres in a 
vaudeville-picture theatre ? 248 



What kinds of screen may the traveling exhibitor 

use to advantage ? 249 

May screen fabric be easily fireproof ed? 249 

Describe one method of stretching a cloth or other 
screen on its frame 249, 252 

Shutter, Rotating 

(See pages Vol. II, and "Protection Optics, Practical.") 

Speed Indicators 

(See Vol. II.) 

Spot, The 

(See "Projection Optics, Practical.") 

Spotlight, The 

Is it difficult to learn the manipulation of a spot- 
lamp? . 259, 260 

Of what does the optical system of the spot lamp 
consist ? 259 

In what way does the projectionist alter the size of 
the spot? 259 

What amperage may be used on a spot lamp ? 259 

What is the "spot" and what difficulty is encountered 
when using the old style apparatus? 260 

How should the carbons and lamp be handled to get 
a clear, round spot ? 260 

May a color wheel or colored slides be used in con- 
nection with a spotlamp ? 260 

Is an A. C. spot practicable and what is the minimum 
amperage for such a spot ? 260, 261 

Is there such a thing as a high intensity spot lamp? 261 

What data must be sent when spot lamp lenses are 
ordered? 262, 263 

What important point must be ascertained b'efore 
ordering lenses ? 263 

What carbons should be used for an A. C spot?... 263 

What focal length lens must be used to obtain a five 
foot spot at different distances ? 264 

Stereopticon, The 

Of what various elements does a single steropticon 
consist ? 442 

Should there be a stereopticon in every theatre pro- 
jection room ? 442 

What is the chief objection to using a motion pic- 
ture projection stereopticon attachment? 442,443 



What is the over-all dimensions of a "slide"? 442 

Why may a stereopticon picture of equal brilliancy 
of illumination with a motion picture be projected 
with a much less amount of light ? 443 

May acceptable stereQpticon pictures be projected 
with lime light ? 443 

Why is it that a cracked condenser lens will show 
in a stereopticon picture and not in a motion pic- 
ture? 443 

What percentage of light is lost if a slide carrier 
be installed in front of a motion picture projector? 

177, 443 

Is it generally bad practice to project motion pictures 
with a slide carrier in front of the condenser? 444 

If a combination projector be used, what precautions 
should be taken to reduce slide breakage from ex- 
cessive heat ? 444 

The Dissolver 

Of what does the dissolving stereopticon consist?.. 445 

In what way is the picture "dissolved"? 445, 446 

What two main advantages has the dissolver over 
the single stereopticon ? 446 

May a single stereopticon be equipped with a semi- 
dissolver ? 446 

How would you proceed to make a dissolving shutter 
for a stereopticon ? 447, 448 

May a dissolving shutter be had where two com- 
bination projectors are used? 448 

Must the projection lenses of a dissolver be perfectly 
matched ? 449 

How would you get the two light beams of a dis- 
solver into perfect register on the screen? 449 

Leveling the Picture 

How would you level the image on the screen ? 449 

Why do stereopticon lens and motion picture lens 
necessary to project an image of the same width 
have different focal lengths ? 450 

Stereopticon Slides 

How may the projectionist make temporary an- 
nouncement slides ? 337, 338 

Of what does a "slide" consist ? 451 

What is a "mat" ? What is its purpose ? 451 

What is the standard slide mat opening dimensions? 451 
Does the present mat opening represent good prac- 
tice ? Explain your answer 451 

What sort of colors are used for coloring slides ? . . 451 



What is the only purpose of the slide cover glass?.. 451 
Which side of slide must go toward light, and why? 451 
How should the slide mat be placed in the slide?.. 451 
How should slides be placed in the slide carrier?... 451 
What is the "thumb mark" and how should it be 
when slide is in carrier? 451 

Handling Slides 

Are perfectly clean slides essential to good results 

on the screen ? 452 

What do dirty slides advertise to the audience?.... 452 

How may slides be quickly cleaned? 452 

How should slides be handled in placing them in the 

carrier ? 452 

What care must be exercised when using a single 
stereopticon ? 452 

Repairing Slides 

What slides may and what ones may not be re- 
paired ? 452 

What extra slide parts should be kept on hand? 453 

Making Advertising Slides 

May slides, to convey messages to the audience or 
to advertise programmes or other things, be made 
conveniently and quickly ? 453 

How may photographic advertising slides be made?. 453 

What precaution must be observed in making photo- 
graphic slides ? 454 

How may a single photographic slide be made, using 
the negative for the slide ? 454 

How may slides be made with inks, or with a type- 
writer ? 455-456 

Supplies, Projection Room 

(See "Projection Room, The") 


What does S. P. S. T. switch, S. P. D. T. switch, 
D. P. S. T. switch, D. P. D. T. switch and T. P. 

S. T. or D. T. switch mean? 92 

For what purposes are various type switches used? 95 

What marking must be on each knife switch? 96 

Must knife switches have certain minimum dimen- 
sions ? 96 

May switches be used for a higher or lower E. M. F. 

(voltage) or amperage than they are marked for? 96 
Are dimension requirements the same for 110 and 
220 volts ? 96 



What is meant by an "enclosed switch"? 94 

Why is it necessary that all switches be enclosed, 
either in an individual covering or in a suitable, 

approved cabinet ? 94 

Why should you avoid installing a knife switch in 
such position that its blades will move downward 

in closing ? 92, 93 

Do some switches have fuses mounted on their bases? 93 
Should switches be so connected that their blades 
will be "alive" or "dead" when the switch is in 
open position ? 94 

Projection Room Incandescent Switches 

What must govern in the matter of switch location 
in theatres ? 94, 97 

What should govern in locating projection room 
switches ? 95 

Should projection room lighting be governed by one 
switch ? 95 

Emergency Lights 

(Also see "Emergency Light Circuits," "Switches" and 
"Emergency Fuses.") 
Should emergency light circuit switches be on main 

switchboard ? 95 

Where should emergency light circuit switches be 

located ? .94, 95 

Of what must you be certain concerning all switches? 96 

Care of Switches 

Name the important points in the care of switches 92 

In inspecting your switches what faults would you 
look for? 96 

What will happen if the contact of switches are 
poor ? 92 

Name the reason why the switch controlling pro- 
jection room incandescent circuits should be lo- 
cated where it may be reached by the projectionist 
from working position 95 


Should projection room knife switches be enclosed 
in cabinets, except the projector table switches, 
of course ? 96 

Is it permissible to use single pole switches in the- 
atres ? 95 

Should projector lamps be connected through a 
double throw switch ? 348 



How may a 4-pole, double throw switch be wired to 
permit one a projector light source to burn through 
a compensarc or other device, while the other 
takes curent through a rheostat for warming up 
the carbons ? 349 

Polarity Changer 

How may a D. P. D. T. switch be used to act as a 
polarity changer or to connect to two sources of 
electrical supply ? 350 

How may a D. P. D. T. switch be used to instantly 
change from one set of fuses to another set? 351 

Stage Switch Marking 

How should stage switches be marked? 106 


(Also see "Switches.") 

What are the main requirements of the National 
Electric Code with regard to switchboard instal- 
lations ? 97 

If the main house switchboar dis located in the pro- 
jection room, what provision of safety should be 
made ? 98 

Why should the main switchboard never be located 
in the projection room unless there be two men 
constantly on duty therein ? 98 

When there are two men constantly on duty in the 
projection room, why is it desirable to locate the 
main switchboard and auditorium dimmers therein? 98 

Give Board Location Careful Consideration 

Why should the main switchboard location have very 
careful consideration by architects and exhibitors? 99 

Examine and describe the electrical action of the 

small panel board, Fig. 14 100 

Locating Connections 

In examining bus bars on switchboards, how may 
you tell where the connections are ? 100 

When examining the main switchboard or the pro- 
jection room main board, you find a circuit con- 
nected to the two outside wires of the 3-wire cir- 
cuit. Is that necessarily wrong ? 102 

If in examining the switchboard you find no screw 
heal where one bus bar crosses under another, 
would you expect there to be an electrical con- 
nection there ? 102 

What forms the keynote to the connections in 
switchboards ? 102 



Built- Up Board 

li, it possible to build up a switchboard from porce- 
lain base switches carrying fuses ? 103 

When a switchboard is built up from porcelain base 
switches, what must they be mounted on and how 
must the board be enclosed ? 103 

What should be placed at the inlet of such a board 
as described ? 103 

Is a board such as is described efficient? 103 

Know Underwriters Requirements 

What should invariably be done before installing 
any stage switchboard in any theatre? 104 

What types of fuse may be used on stage switch- 
board and what type is prohibited ? 106 

What various things should the stage switchboard 
carry ? 106 

What especial reasons are there why stage switch- 
board equipment should be regularly inspected and 
kept in the best possible condition ? 106 

Should one man be in complete charge of the stage 
switchboard ? 106 

Should a stage switchboard always be wired from 
the back? 106 

What authorities should be final with those con- 
templating installing a stage switchboard? 106 

What should govern in placing the projection room 
switchboard ? 342 

Temperature, Maximum Permissible 

(See Vol. II) 


(See "Projection Distance," under "Projection Room, 


(See "Projection Room, The") 


(Page 596, Vol. II) 


(See "Projection Room, The") 

Voltage Drop 

(See "Projection Room, The," and "Wire Systems ") 



(See the "Projection Room" in question list and pages 
317 to 319 of text.) 

Wire Systems 

Of what does an electric "circuit" consist? 3 

Is a 3-vvire "circuit" really a single circuit? 3 

How is power conveyed from an electric generator 
to distant points ? 4 

What do the circuit wires in your theatre really 
represent ? 4 

Does electricity seek to escape from the power lines 
to the earth? 5,51,352 

Has the positive wire of an electric circuit any elec- 
trical affinity for anything except the negative of 
the same power source ? 6, 51, 352 

Were you to make an electrical connection between 
the positive of one power source and the negative 
of another power source not electrically connected 
to the first, what would happen ? 6 

Assuming an electrical power source and all its con- 
nections to be thoroughly and completely insulated 
from earth (a condition seldom if ever found), 
what would happen were either its positive or its 
negative wire to fall to wet earth ? 6 

Does anything pass over, along or through a wire 
over which electrical energy — power — is being con- 
veyed ? 52 

What Lowers or Raises Resistance 

With a given wire diameter what will cause the re- 
sistance of a circuit to increase or decrease? . .53, 60 

What two elements must we know in order to cal- 
culate the number of amperes flowing through a 
circuit ? 54 

When is the normal capacity of a water pipe or 
electric conductor said to have been reached?.... 62 

How does increasing or decreasing the diameter of 
a water pipe or electric conductor affect its resist- 
ance, assuming the flow of water or current to 
remain constant ? 63, 64 

Name the various things which will increase or de- 
crease resistance in a water pipe or in an electric 
circuit 63, 64, 68 

Has E. M. F. (voltage) any effect on the size wire 
required to carry a given number of amperes? 63 




What two elements determine the amount of elec- 
trical power which may be carried by a wire of 
given diameter without overload? 14 

At what point does overload begin in an electric 
conductor ? 64 

No Overload 

Name two important reasons why an electric circuit 
must never be overloaded 64 

Do different metal offer different degree of resist- 
ance to electric current ? 65 

Does the resistance of all metals used for electric 
wires increase as their temperature increases? 65 

To what is the increase or decrease of resistance in 
electric conductors due to changes in temperature 
proportioned ? 66 

Very Necessary- 
Is it necessary that the projectionist be able to cal- 
culate the resistance of copper circuits ? 68 

Does the resistance of copper wire remain constant 
up to a certain point in amperage load? 68, 71 

The Economic Limit 

What is the limit, from an economic standpoint, of 
resistance in any electric circuit ? 69 

Is it possible for the projectionist to apply his knowl- 
edge of voltage drop due to resistance in practice? 

69, 74 

What safe guide have we in the matter of wire 
capacities ? 69 

With what does table No. 1, page 70, supply you?... 69 

What instrument is standard in the United States 
and Canada for measuring the diameter or size of 
copper wires ? 69 

Why are rubber covered wires rated at less capacity 
than exactly similar wires having different insula- 
tion ? 83 

How would you apply table No. 2, page 71 ? 69 

To Find Voltage Drop 

Having ascertained the resistance of a circuit with 
the assistance of Table No. 2, how would you then 
proceed to find the voltage drop? 69, 71 

What is the length, in fractions of an inch, of one 
"mil"? 70 

May varnished cloth insulation be used on wires 
smaller than No. 6 ? 70 



How would you find the amperage capacity of an 
aluminum circuit ? . . , 70 

What is the smallest wire recognized by the Na- 
tional Board of Fire Underwriters for electric light 
or power circuits ? 70 

Minimum Size Wires 

What is the smallest size wire we are permitted to 

use for interior circuit wires ? 70 

Why is it that the resistance of a wire remains con- 
stant until it is loaded beyond its capacity? 71 

The Mil Foot 

What is meant b ythe "Mil Foot Standard of Re- 
sistance" ? 73 

How would you proceed to calculate the resistance 
of a copper wire o fany given size and length?.. 73 

Calculating Resistance 

What resistance will a 100-foot circuit of No. 4 
copper wire offer at any load up to full load?.... 73 

What would be the resistance of a copper circuit of 
No. 6 wire, if the circuit be 75, 50 or 90 feet long? 

73, 74 

Note : When we speak of a "circuit" the length of 
the circuit itself is meant. The wire length will 
be twice that length, if it be a 2-wire circuit. 

W T hat is the formula for calculating the resistance 
of a copper circuit when size of wires, length of 
circuit and amperage are known ? 73, 74 

What is the formula for calculating the voltage drop 
of a circuit, when length of circuit, size of wires 
and amperage is known ? 75 

Suppose you have a projection room feeder circuit 
90 feet in length. It is to have capacity to carry 
a total of 125 amperes, but its usual, normal load 
will be 60 amperes. What size wires would you 
recommend ? 76 

In what is the cross section of wires measured? 77 

Circular Mil 

What is a circular mil (CM.) ? 77 

What size wire would have a cross section area of 
one mil ? 77 

Calculating Area of Cross Section 

To what is the area of cross section of a round wire 

proportional ? 77 

What is meant by ''squaring the diameter"? 77 



If a wire have a diameter of 20 mils, what would be 

its area of cross section in C. M. ? 11 

What is the diameter, in mils, of a wire J4 mcn m 

diameter ? 11 

What is the diameter, in mils, of a wire Y% inch in 

diameter? 11 

What is the diameter, in mils, of a wire Y% i ncn m 

diameter ? 11 

What is the area of cross section of each of the 

wires named in questions Nos. 000,000 and 000? 11 

How may you readily ascertain the capacity of any 

round wire ? 11 

Wire Gauge 

Describe the American Standard (P. & S.) wire 
gauge 78 

What other tool is available, besides the wire gauge, 
for accurately measuring wire diameters? 78 

Wire Systems Likely to be Used 

There are two wire systems the projectionist is likely 
to be called upon to handle. What are they?.... 84 

Is it practical to connect a projection arc to a series 
arc system ? 84 

Should an attempt be made to connect a projection 
arc to a series arc system what is likely to happen? 84 

Two-Wire System 

Describe and draw a sketch of a multiple arc, or 
2- wire system 84 

May a projection arc be connected to a 2-wire sys- 
tem operating at commercial voltage at any point? 84 

What various things must one know before connect- 
ing a projection arc to a 2-wire system? 85 

What must be connected in series with an arc lamp 
connected to a circuit before it is put into opera- 
tion? 85 

What is meant by "commercial voltage" as used in 
question No. 203 ? 85 

Three- Wire System 

What is the most widely used system for the distribu- 
tion of electric light and power ? 85 

Upon what basic principle does the 3-wire system 
operate? 85 

What, in effect, is a 3-wire system? 85, 87 

Are there two kinds of 3-wire systems ? 354 



If two 110-volt generators be connected in series, 
what will be electric pressure — voltage — between 
the two outer wires and between either outer wire 
and the center, neutral wire ? 86 

In a 3-wire circuit why do you have two times as 
much electric pressure — voltage — between the two 
outer wires as you have between either outer wire 
and the center one ? 86, 87 

True Positive and Negative 

What is meant by the "true positive" and "true 
negative" of a 3-wire system or circuit? 86 

Is the neutral of a 3-wire circuit both positive and 
negative ? 86 

Why is the neutral wire of a 3-wire circuit both 
positive and negative ? 86 

Why is the 3-wire system so much used? 87 

Explain the electrical action indicated in Fig. 10 87 

Apparatus Works in Series 

Name the peculiarity of the 3-wire system which is 
the real cause of its high efficiency 87 

Balanced Load 

What is meant by a "balanced" 3-wire circuit or 
system ? 88 

Does a state of perfect balance in load often, exist? 88 

Why is a balanced load always desirable ? 88 

Why is a balanced load especially desirable with a 
small, heavily loaded 3-wire system ? 88 

If a rheostat be used to control the current for an 
arc, is there any advantage in connectiong the arc 
to the outside wires of a 3-wire system? 88 

Is there advantage in connecting to the outside wires 
of a 3-wire system if a motor generator, a mercury 
arc rectifier or an economizer is used? 89 

Explain the electrical action as per Fig. 11 90 

Local Effect of Unbalanced Load 

What would be the effect if your theatre load be 
unbalanced (3-wire circuit) and the neutral fuse 
blows ? . / 91 

How may you determine the amount of unbalanced 
load to your theatre 3-wire circuit ? 91 

How would you proceed to figure the wire sizes for 
a 3-wire circuit to carry any given amperage? 91 



Neutral Grounded 

Is one wire of an Edison 3-wire system always 
grounded ? 353 

You connect a 2-wire circuit to each side of a 3-wire 
circuit, and to each circuit so considered you con- 
nect a 60-watt incandescent lamp, and nothing else. 
What is the effect. How much current will flow 
in the neutral ? 87 

Connect a 10-ampere motor to one side of a 3-wire 
circuit and lamps using six amperes to the other 
side. How much current will the neutral fuse 
carry ? Fig. 10 and page 86 

If 110- volt lamps using 30 amperes be connected to 
one side of a 3-wire circuit, and motors using 30 
amperes be connected to the other side, what will 
the effect be? How much will the neutral fuse 
carry ? Fig. 11 and page 90 

If each of the three wires of a projection room feed 
circuit be fused at 60 amperes, and apparatus using 
30 amperes be connected to one side, and to the 
other side apparatus using 25 amperes, could you 
connect a motor or arc using 30 amperes without" 
changing any of the fuses? Fig. 11 and page 90 

Wire Terminals and Wire Splices 

Poor Splices, or Dirty or Loose Wire Terminals Mean 
Unnecessary Resistance, and Unnecessary Resistance 
Means Waste and Trouble. 

Should wires have terminal lugs where they connect 

to apparatus ? 121 

How should terminal lugs be attached to the wires? 121 

How to Solder a Lug 

Describe the process of attaching a terminal lug to 
a wire 121 

Are there special terminal lugs for hot connections? 124 

How do hot terminal lugs make contact with the 
wire ? 124 

Why is it essentially important to have the metal 
of the wire and of the lug perfectly clean before 
soldering? 121 

Have Contact Clean 

In attaching terminal binding posts of apparatus 
what should you be very careful about ? 122 



What almost invisible thing may there be on metal 
of lug or binding post, or both, which, if not 
cleaned off, will offer high resistance ? 122 

Is all energy wasted in overcoming resistance of 
poorly made wire splices or poor connections be- 
tween lugs and binding posts registered on the 
meter ? 122 

Removing Insulation 

Describe the correct process of removing insulation 
from wire ends preparatory to making splices.... 123 

What serious weakening of the wire may you cause 
if you remove the insulation wrongly? 123 

What provision do Underwriter's rules make con- 
cerning the mechanical and electrical perfection 
of a splice before soldering ? 123 

How to Solder Wire Joint 

Describe the process of soldering a wire joint 123 

What care must be exercised in making a solder 
joint ? 123 

How must a splice be insulated after it has been 
made ? 123 

What is the best method of making a splice in as- 
bestos covered wires ? 123, 124 

Stranded Wire and Connectors 

What must be done b'efore a wire connector may be 
used on stranded wire ? 124 

Name the constituent parts of an acceptable solder 
flux 124 


Projection in the Modern Moving 

Picture Theatre and What 

It Entails 

MODERN motion picture theatres are indeed temples of 
beauty. In many cases they represent an outlay 
reaching well into six figures, the income from which 
depends very largely upon the excellence of what paying pa- 
trons see upon the screen. Great producing companies have 
established reputations which have drawing power at the box 
office. The same is true of what we term ''stars." The expert 
work of cameramen, and those others who contribute to the 
truly wonderful photographic results found in modern films, 
all have their share in popularizing the motion picture as a 
salable form of theatrical amusement. A good orchestra has 
considerable drawing power. 

Fine seats, good ventilation, beautiful light effects and deco- 
rations all lend aid in the sale of tickets, but the fact remains 
that even though a theatre have all these things, still, if there 
be anything less than high class, expert work in the projec- 
tion room, the shadow forms of the artists will not appear to 
best advantage, the photography, though wonderfully beauti- 
ful in the film itself, will be only ordinary on the screen, and 
in many other ways the show will be made less pleasing, with 
the result that the box office income will inevitably suffer. 

The following is put forward as a flat statement of amply 
proven fact: Given a free hand, unhampered by unreasonable 
schedule restrictions, limited amperage or penuriousness in 
the matter of projection room operating expense, the care- 
ful, painstaking projectionist who is equipped with expert 
knowledge of his profession, can "put over" a production of 
mediocre merit, sending forth an audience at least fairly well 
pleased and of mind to come again; whereas the slovenly, 
careless projectionist, or the projectionist not equipped with 
expert knowledge, although otherwise equally unhampered, 
will either cause the same subject to fall flat, or will give a 



less satisfactory performance with a production of far 
superior merit; and since inferior screen results must inevi- 
tably react unfavorably on future ticket sales, it follows that 
careful work and expert projection knowledge has direct value 
to the box office through increased patronage of the theatre. 

Not only is this true, but the well informed projectionist is 
in a position to effect material saving in projection room ex- 
penditures, both in the matter of daily operating expense and 
in better and longer service of equipment. This latter item 
may be a very large one indeed, if we include the possible 
saving in film damage through intelligent adjustment of the 
machine tensions and intelligence in the matter of rewinding 
and handling of film, remembering that all film damage must, 
in the last analysis, be charged back to the theatre in the 
form of increased film rentals made necessary by added over- 
head expense to exchanges through frequent purchase of 
prints to replace those ruined by unintelligent handling. 

With these facts in view we may readily understand the im- 
portance attached to the study of the details of his profession 
by the projectionist, and this book is designed primarily to 
supply detailed information, in plain words and understand- 
able form, concerning those many things the competent, 
modern projectionist should and must know. 

We shall labor hard to produce the best book possible, 
but have no hope of attaining perfection. The work is a large 
one and it is inevitable that some errors — minor ones only, we 
trust — will be found in its text. These we ask you to view 
with charity, remembering that few things in this world of 
ours are perfect. 

This book, like its predecessors, is designed for the use of 
practical men, hence, as in past editions, we shall pay very 
much more attention to practical things and understandable- 
ness, than to absolute technical correctness. Strict technical 
correctness, especially in matters electrical and optical, often 
involves the use of a maze of words and technical terms, many 
of which latter could not possibly be understood by the 
ordinary man without such long-winded explanations that the 
novice would become confused and discouraged. 

Therefore, when we are able to make a point sufficiently 
clear for all practical purposes merely by the sacrifice of some 
unimportant point of technical correctness, we shall unhesi- 
tatingly do so, believing that course to be, all things consid- 
ered, best. 


Electrical Action 

IN order to arrive at a comprehensive knowledge of elec- 
tricity, one must first understand the underlying principles 
which govern its action. It is absolutely imperative that 
the projectionist have at least a good working knowledge of 
electrical action, because he will be put in full charge of ap- 
paratus for generating and using current, which devices will 
operate safely and with high efficiency, or with low efficiency 
and perhaps unsafely, exactly in proportion to the expert skill 
and knowledge he is able to make use of in their adjustment, 
care and handling. 

We will first try to convey an understanding of the one 
basic, underlying principle upon which all electrical action 
is based, always remembering that electricity and magnetism 
are two entirely separate and distinct things, notwithstanding 
statements of some authorities to the contrary. 

POLARITY. — Polarity is the very foundation principle 
upon which all electrical action is based. Precisely what 
electricity is, no living man knows. Eminent scientists differ 
widely in their views as to the cause of the phenomenon. Some 
authorities claim it to be a "molecular bombardment," while 
others hold it to be something entirely different. With such 
arguments the practical man has little interest. At best they 
represent little more than abstract theories. They have no 
importance as applied to the work of projection. 

We may not know the precise nature of the thing which 
does it, but we do know that if we touch a "live" positive wire 
to a negative wire attached to the same generator, there will 
be a flash and a shower of sparks. We also know that by 
Connecting these two wires through certain devices, such as 
motors and lamps, instead of an uncontrolled flash and shower 
of sparks we can and do get light, heat or power. In other 
words we can make the electric force work for us in its pas- 
sage from positive to negative. 

So far as electric action is concerned, every electric circuit 
consists of just two wires — a positive and a negative. True, 
there may appear to be more, as in the three-wire system, but 
when analyzed we find that the additional wire or wires 
merely operate to form additional complete circuits, which 


may either be used singly or together, as will be fully ex- 
plained in the proper place. 

One positive and one negative conductor constitutes what is 
called a "circuit." Every electric generator (dynamo) and 
every battery has a "positive" and a "negative" pole. In order 
that the power of the generator or battery be available for 
use at a distance, we attach a wire to each of these poles. 
These wires become a part of and represent the poles of the 
generator or battery, so that connecting a lamp or motor 
to them at any portion of their length is the same as attach- 
ing it to the actual poles of the machine itself, modified only 
by the fact that resistance is offered by the wires to electric 
action which for a given size of wire increases as the length 
of the wire increases, as will be fully explained under the 
proper heading. 

When disconnected from the generator, or when the gen- 
erator is not running, these wires are precisely the same as 
any ordinary wire. They are "dead." But the instant they 
are connected to the poles of a working generator or battery 
they become "live wires," the positive wire becoming charged 
with an electrical energy which, measured in "volts," is 
called "voltage" and corresponds to pressure in a steam boiler. 
Steam confined under pressure in a boiler seeks to expand 
its volume, since by so doing its pressure is reduced. When 
it escapes into the open air its pressure is reduced to at- 
mospheric pressure, hence it seeks always to so escape. 

Electricity under pressure, or tension, in the positive wire 
seeks to escape into the negative wire for precisely the same 
reason — its pressure or tension is reduced to zero by so doing. 

Technical electricians will no doubt feel inclined to criticise 
our last statement, but while they may do so from the purely 
technical standpoint, what we have said describes what 
actually apparently takes place in practice, and it is under- 
standable. Those who care to go into fine-spun theories will 
find books in plenty which will carry them as far as they may 
wish to go into a very maze of it, but when they have done, 
they will only be able to tell us, in technical language, what 
for all practical purposes amounts to precisely what we have 
just said. Our concern is to make you understand the 
practical effect of what takes place between positive and 
negative. If we accomplish that result we are well satisfied. 

The affinity of positive for negative — the desire, if you 
please, of the positive energy to become negative — is what we 
term "polarity." It represents difference in electrical pres- 


sure as between positive (-f ) and negative ( — ). It is measured 
in volts. 

HOW WORK IS PERFORMED.— Steam under pressure 
generated by confining it in a boiler, seeks to lower its pres- 
sure by expanding in volume. We allow it to enter the cylinder 
of a steam engine in which is a movable piston. On one side 
of the piston is the pressure of the steam, and on the other 
only the pressure of the air, which for our purpose represents 
zero. In seeking to expand its volume, and thus reduce its 
pressure, the steam will shove the piston ahead of it to the 
end of the cylinder, pulling with it the load attached to it, 
thus generating power. 

In doing this the steam itself is not consumed. It still exists, 
having been discharged into the open air, but its pressure has 
been consumed. Steam is merely the medium, the compres- 
sion of which stores up power. It acts precisely as does a 
coil spring. Compress the spring and you will have stored- 
up power, which will be available until the spring has again 
expanded to its former state, whereupon, while the spring 
itself remains, the power has all been expended and is gone. 

We cannot see electricity. The light we see in a lamp is 
not electricity itself, but a product of its power. We cannot 
weigh it. Apparently it has no weight. We cannot 
feel it, except in the form of a "shock," which again is not 
electricity itself but a product of its power. We do, however, 
find its action to be almost precisely the same as that of steam 
or water under pressure, so that we may readily use these as 
a basis for comparison. 

Apparently, as already set forth, electricity exists under 
pressure on one wire, the positive, and apparently it loses 
its pressure in the act of entering the negative wire — the 
act of becoming negative — hence, since pressure is power and 
pressure is consumed in passing from positive to negative, it 
follows that power is generated when current passes from 
positive to negative and this power is made available for use 
by means of what amounts to an electric engine, one side 
(pole) of which is connected to positive and the other to 
negative. The particular power generating device may be a 
lamp, by means of which light is produced, a motor by means 
of which pulling power is made available, or it may be a 
heating coil. In either case electricity is made to do a useful 
thing, hence its power is turned into useful channels. 

mistaken idea entertained by many, that electricity seeks to 


escape from the wires into the earth. This is not true, except 
insofar as the earth offers a path for the current from positive 
to negative. Let it be clearly understood that : 

There is absolutely no electrical affinity of the positive wire 
of a battery or dynamo for anything else except a wire at- 
tached to the negative pole of the same battery or generator, 
except in cases where two or more batteries or generators 
are so connected that they, in effect, form one power source. 

Set two separate 5,000 volt generators operating, and you 
may, with perfect safety, bring the positive of one into direct 
contact with the negative of the other. The result will be 
exactly the same as though two dead wires were brought into 

Thoroughly insulate a 10,000 volt generator from the 
ground, including all the wires and apparatus attached to 
it, and you may with perfect safety stand with your bare 
feet on wet ground and handle either the raw (uninsulated) 
positive or negative wire separately. But if the positive or 
negative be "grounded," and you touch the wire of opposite 
polarity, the current would leap through your body into the 
ground and through the ground into the other wire. It does 
not necessarily follow, however, that when both wires have 
current carrying connection with the ground, the current will 
always pass from positive to negative, because the "ground," 
as it is called, may have such high resistance that the pres- 
sure cannot force the current through. 

percentage of theatres the projectionist is placed in direct 
charge of an electric generator. It is therefore essential that 
he not only understand its handling, adjustment and care 
from the mechanical viewpoint, but that he also have accurate 
knowledge of its electrical action and the theory upon which 
it acts in the generation of electric energy, since 

OUGHLY UNDERSTAND, and the more a man knows about 
the thing he is handling the better service he can cause it to 

Fill a glass jar of any convenient size two-thirds full of 
water, to which add ordinary sal ammoniac, procurable at any 
drug store, in the proportion of one pound to the gallon of 
water. In this solution suspend a sheet of copper having an 
area of say twenty square inches on each of its two sides. Near 
to, but not in actual contact with the copper, suspend a piece 


of zinc of approximately the same superficial area, and the 
whole will constitute the simplest form of electric generator 
known, the assemblage constituting what is known as a "wet" 
battery. If we make electrical connection between the copper 
and zinc of such a battery, current, generated by chemical 
action, will flow from copper to zinc, the former being posi- 
tive (-f) and the latter negative ( — ). A well proportioned 
battery of this sort will generate about one volt pressure and 
several amperes of current while it lasts. 

In theory it would be possible to join sufficient of these bat- 
teries to produce almost any desired voltage and amperage, 
but in practice this would be impractical. The use of gen- 
erating batteries is almost entirely confined to light work, 
such as the ringing of bells and buzzers, the telegraph and 
like service where comparatively little energy is required. 

For power purposes we depend upon the dynamo, or 
"generator," as it is usually termed. The dynamo depends for 
its action primarily upon magnetism, and the generation of 
electric energy in the armature of a dynamo is based upon 
the following law : 

"If an electric conductor in the form of a closed circuit be 
moved in a magnetic field in such a way that lines of force 
are cut, a current of electricity will be generated therein, 
which same will flow in a direction at right angles to the 
line of motion/' 

See Hawkins' Electrical 
Guides, Vol. 1, Pages 125 
to 136, for a detailed ex- 
planation of this law and 
its operation. 

In Fig. 1 we see the 
diagrammatic representa- 
tion of the simplest pos- 
sible form of electric 
dynamo. N and S are re- 
spectively the north and 
south poles of a perma- 
nent magnet, between and 
around the poles of which 
flow magnetic lines of 
force, represented by the 
dotted lines. Within the 
magnetic field thus formed 
is copper wire A — B, bent 

Figure 1 


as shown, one end being attached to flat-faced metal ring C, 
and the other end to a similar ring, D. On the face of each 
of these rings, and in electrical contact therewith, rest metal 
brushes J and K. Wires E and F, which represent an outside 
circuit, connect respectively with brushes J and K. It will 
thus be seen that wire coil A — B, through ring C, brush K, 
wire E, lamps L, wire F, brush J and ring D, forms a com- 
plete electric circuit. 

HOW ARMATURE WORKS.— Now if by means of crank G 
we rotate coil A — B in the direction indicated by arrow O, 
coil side A will pass down and side B up through the mag- 
netic field, cutting across lines of magnetic force in so doing. 
This will (see law before quoted) generate an electric im- 
pulse (current) which will, under the conditions, flow through 
the coil, rings, brushes and circuit in the direction indicated 
by arrows M and P. 

ARMATURES GENERATE A C— And now a step further. 
In Fig. I we assume crank G to rotate in the direction indi- 
cated by arrow O. Under that condition the current will flow 
toward brush K, but as the coil is rotated the electric impulse 
becomes weaker, since the coil sides travel more nearly in 
the direction of the lines of force, hence cut less of them, until 
finally side B stands directly over side A, and both are travel- 
ing in the same direction as the lines of force, therefore, cut- 
ting none of them, so that there is no electric impulse 

At this point the wires are "dead." There is no voltage, 
hence no current. But as we rotate the coil still further, the 
wires again begin to cut across the lines of force, and the 
electric impulse is revived and becomes stronger until side 
B reaches the position formerly occupied by side A, when 
the voltage is again at maximum. But since the current 
always flows in the same direction with relation to the magnet 
poles (see arrows P and M) its direction has now been re- 
versed in the coil itself, and it flows into brush J, instead of 
brush K, hence around the circuit in the opposite direction. 

Remember that the current within the magnetic field always 
flows in the direction of arrows P and M and that the sides 
of the coil are constantly exchanging their positions as the 
coil is rotated, hence the current in the coil and circuit is 
reversed with each half turn of the armature, or in multipolar 
machines, every time a coil passes through the field of one 
of the magnetic poles of the machine. 


ing continuously in one direction, called "Direct Current" 
(D. C), is obtained from armatures which produce alternating 
current (A. C.) by means of what is termed "commutation," 
as follows, it being understood that we only intend to explain 
the principle involved. To set forth in detail the intricacies of 
of actual practice would require many pages of text, as well 
as many very complicated illustrations. 

Figure 2 

In diagram A, Fig. 2, we see armature coil A — B and a com- 
mutator ring split into two sections, E and F, to each section 
of which an end of coil A — B is attached. Resting upon and 
in electrical contact with the "commutator" thus formed, are 
brushes C and D, to which outside circuit G is connected, as 

Remembering what has already been said about the direc- 
tion of flow of the current in armature coils, and assuming 
it to be in the direction of the arrows, we readily see that 
under the condition shown in diagram A it will flow into com- 
mutator section E, out to the circuit through brush C, and 
back through brush D. But when the coil has been rotated 
until coil side B is in position formerly occupied by side A, 
as shown in diagram B, the current will then flow into com- 



mutator section F instead of commutator section E; but since 
brush C now rests on commutator section F instead of E, the 
current, while reversed in the coil itself, will still flow in the 
same direction over the outside circuit, so that we shall have 
"direct current" everywhere except in the armature of the 
dynamo itself. 

This is called "commutation." Please understand that in 
Fig. 2 we illustrate the principle involved in commutation only. 
In modern dynamo armatures there are many coils and com- 
mutator sections, bars or segments, as they are variously 
called, but regardless of complications the principle involved 
is exactly what we have described. See Hawkins* Electrical 
Guides, Vol. 1, pages 171 to 180, for a detailed description of 
the action. 

Figure 3 


We believe a careful study of the foregoing will enable 
our readers to understand how current is generated (we use 
the word "current" for convenience; electromotive force would 
be more nearly technically correct, but current is expressive, 
readily understood and short) in a dynamo, how it gets from 
the armature to the outside wires, and the method by which 
it is changed from A. C. to D. C, which is all we may reason- 
ably be expected to accomplish along these lines in the lim- 
ited space available for a discussion of the subject in a work 
of this kind. Students desiring to examine into the matter in 
greater detail may do so by consulting the references we have 
provided. Matter helpful to an understanding of commuta- 
tion will also be found in Hawkins' Electrical Guides, Vol. 2, 
pages 237 to 243. 

In studying commutation one very important thing to 
remember is that in practice all armature coils are intercon- 
nected with each other through the commutator. Unless this 
point is understood, the student is apt to be sadly puzzled 
as to how there can be any connection between positive and 
negative brushes located at opposite sides of the commutator, 
when the ends of individual coils may connect to adjoining 
commutator bars. 

Fig. 3 illustrates what is known as a "two pole, direct cur- 
rent, shunt wound generator," or dynamo. N and S indicate 
respectively the north and south poles of field magnet A. 
F — F indicates the field winding, which we see coiling around 
the upper or bowed part of the magnet. B is the armature 
around and through which pass lines of magnetic force gen- 
erated by the field magnet. C is the commutator, D — D the 
brushes, E — E the wires leading to the outside circuit and G 
the movable lever by means of which the "field rheostat" is 
adjusted. 1, 2, 3 and 4 are the coils of resistance wire forming 
the "field rheostat/' 

The voltage and capacity of the machine described will 
depend, within certain limits, upon (a) the number of lines of 
magnetic force passing through the armature, or in other 
words, the strength of the magnetic field, or in still other 
words, the density of magnetic flux per square inch of area 
of the surface of the pole pieces of the magnet which lies 
next the armature, (b) Number of turns to each armature 
coil and number of coils the armature carries, (c) Rotary 
speed of armature, each of which items has directly to do 
with the number of lines of magnetic force which will be 
cut per second. 


Of course, it is understood that other things, such as the size 
and winding of the magnets, kind of armature core et cetera, 
have much to do with the ultimate performance of the ma- 
chine, but we are merely explaining to you the principle upon 
which the generator operates, which, with some variations in 
methods, is always the same. 

magnet of the type of dynamo shown in Fig. 3 is a "perma- 
nent" magnet, meaning that it does not become entirely 
demagnetized when the armature comes to rest. In effect, 
when lying idle it is just an enormous horse-shoe magnet, much 
the same, except for its size, as the horse-shoe magnets that 
children play with. The magnetism retained when the machine 
is at rest is called "residual magnetism." It is very much 
too weak to enable a dynamo to build up and maintain a com- 
mercial voltage. The most we might hope to accomplish by 
its use would be to generate perhaps ten volts' pressure. 

Examining Fig. 3 we see that wire F — F coils around the 
upper part of magnet A. It may connect either directly to 
brushes D — D or to lines E — E a short distance from them. 
In other words, coil F — F and the armature form a complete 
circuit, which but for field resistance H would be a short cir- 
cuit, and is in fact a short circuit when lever G is in position 
shown. Coil F — F forms what is known as the "shunt field 
circuit" and the generator shown is a "shunt wound" ma- 

It is a well known fact that if a current of electricity is 
passed through a wire wound upon a magnet in the way wire 
F — F is wound, the magnet will have its power increased, and 
that the power of the magnet will increase proportionately 
as the current is increased until the point of "saturation" 
(iron is said to be saturated with magnetism when the point 
is reached where it will receive no more) is reached. 

Bearing the foregoing in mind, a dynamo starts generating 
electro-motive force as follows : First having placed lever G 
in the position shown, which "cuts out" or eliminates all the 
resistance of the field rheostat, the switch connecting wires 
E — E with the outside circuit is opened, so that all current 
generated must flow around shunt circuit F — F, there being 
no place else for it to go. Power from an engine or motor 
is now applied and armature B is rotated at high speed, its 
coils cutting lines of magnetic force in the weak field of 
residual magnetism. This immediately creates a slight electro- 
motive force, the current resulting from which flows around 


shunt field F — F, thus slightly strengthening the magnet, 
which instantly increases its magnetic flux so that the arma- 
ture wires cut more lines of force, which in turn strengthens 
the current flowing over the shunt field. 

This process continues until the normal voltage of the ma- 
chine is reached, whereupon the switch connecting with the 
outside circuit may be closed, thus connecting the machine 
with its load, and lever G adjusted, until the resistance of the 
field rheostat limits the shunt field current flow to the value 
necessary to maintain the magnetic field at the strength re- 
quired to maintain the desired voltage. 

Modern dynamos for the most part have more than two 
pole pieces, but only two poles, nevertheless. The added po!e 
pieces merely serve to enable the generator to produce a given 
amount of electrical energy at a given frequency with a lower 
armature speed and a less massive construction. 

rent, also known as "continuous current" (though the term is 
not always correctly applied. See definition p. 25) and com- 
monly abbreviated as "D. C," acts or flows continuously in 
one direction. It is commonly considered as flowing from 
positive to negative. In theory the electric impulse, commonly 
referred to as "current flow," is outward from the positive 
brush of the generator, or positive pole of the battery, on 
one wire of the circuit, along that wire (positive wire because 
it is attached to the positive brush or pole) to and through 
the various lamps, motors, et cetera to the negative wire 
(negative because it is attached to the negative brush or 
pole of the generator) and along that wire back to the nega- 
tive pole of the dynamo or battery. 

Alternating current, commonly referred to by the abbrevia- 
tion "A. C," is the current normally generated in the dynamo 
armature sent out on the circuit without commutation, so that 
the current on the entire system reverses its direction exactly 
the same as it does in the dynamo armature. 

WHY A. C. IS USED.— Knowing that D. C. is best for pro- 
jection, and equally good or even better for incandescent 
lighting, and that it may be used for power purposes, the 
novice very naturally inquires why it is not used exclusively. 

There are several reasons why A. C. is used, three of which 
are as follows : First, it is not deemed practical to commu- 
tate the current from the armature of a dynamo the voltage 
of which exceeds 500, because of the difficulty of insulating 
the commutator bars. 


The second reason is that since wattage, which is the meas- 
ure of electrical power, is the product of volts multiplied by 
amperes, the size of a wire necessary to convey a given 
number of horsepower will be much less at high voltage, thus : 
Suppose we wish to transmit 10,000 watts (746 watts equal one 
horsepower) at 100 volts. Since volts times amperes equals 
watts, it follows that watts divided by volts equals amperes ; 
hence, to transmit 10,000 watts at 100 volts would require 
10,000-^-1 00= 100 amperes, and the power transmitted would be 
about 13.5 horsepower. But if the voltage were 1,000 instead 
of 100, then only 10,000-H, 000=10 amperes would be required 
to convey the required 10,000 watts of power. 

To put it another way, 100 amperes at 100 volts represents 
exactly the same wattage (horsepower) as does 10 amperes at 
1,000 volts. To carry 100 amperes requires a No. 3 wire, 
whereas 10 amperes can be carried by a No. 16 wire ; and 
since a No. 3 wire is .22942 and No. 16 wire .050820 of an inch 
in diameter, it is readily seen that with high voltage a given 
wattage (horsepower) can be conveyed on very much smaller 
wires than could be used were a lower voltage employed. See 
Hawkins' Electrical Guide, No. 4, page 997, for further 

Third, still another factor entering into the matter is the 
fact that once the current has been generated and commuted 
into D. C, its pressure (voltage) cannot be raised or lowered 
except by the use of expensive machines having moving parts, 
thus requiring more or less constant attention, whereas it is 
quite practicable to attach a very simple device known as a 
"transformer" (See page 596), which has no moving parts 
and therefore requires practically no attention, to A. C. lines 
at any desired point, the action of which will be to raise 
the voltage to any desired pressure which it is commercially 
possible to insulate, or to lower it to any required voltage. 

It therefore follows that A. C. may be generated at relatively 
low voltage, "stepped up" by means of transformers to any 
required pressure, transmitted for long distances over relative- 
ly small wires and again "stepped down" to commercial pres- 
sures at destination. Or power for commercial purposes may 
be generated at high voltage, which may be "transformed" 
to a voltage to suit any commercial requirement at any desired 
point along the lines. 

There are other reasons why A. C. is more desirable for 
general commercial use than D. C, but those named are 
perhaps the chief ones. 


plained, A. C. constantly reverses its action, flowing in one 
direction for a small fraction of a second, and then in the 
opposite direction for an equal period of time. Commercial 
current now in general use in the United States and Canada 
seldom exceeds 60 cycle and is seldom lower than 25 cycle. 
Taking 25 cycle current for example, the current would flow 
in one direction for l/50th of a second and then in the oppo- 
site direction for l/50th of a second. Each of these periods is 
called an "alternation," and the two periods together represent 
what is known as a "cycle." (See definition of "cycle," Page 
26.) If the periods of flow were l/120th of a second, then 
the current would be called "60 cycle," because there would 
be 60 complete cycles (120 alternations) per second of time. 
With a 25-cycle current there are twenty-five complete cycles 
— fifty alternations — per second. 

Electric dynamos may be built to produce almost any de- 
sired number of cycles per second (called "current fre- 
quency"), but the two standard frequencies used almost 
universally for commercial work are 25 and 60 cycles per 
second. The first named is employed where the current is 
to be converted to D. C, as in the case of street railways, 
and where the current is to be used mostly for power pur- 
poses. Sixty cycle current is used almost universally where 
the current is to be used extensively for both lighting and 

Low frequency current (25 cycle) is objectionable for light- 
ing for the following reasons : If a light be produced by very 
low frequency current there will be a perceptible flicker, due 
to the fact that the E. M. F. sinks to zero twice during each 
cycle, with a consequent dimming of the brilliancy of the 
light, but as the frequency becomes more rapid, the eye is 
unable to follow the rapid changes of brilliancy and the light 
appears to be steady. Due to this cause, 25-cycle current 
has a decided flicker, whereas, insofar as concerns the ability 
of the eye to detect it, 60-cycle current has none at all. 

It is well that the projectionist have at least a fair under- 
standing t>f these various matters, because he is placed in 
direct charge of motors and generators of considerable ca- 
pacity, and of projection light which may be seriously affected 
by low current frequency. Moreover, in some places and 
under some circumstance, problems allied to projection may 
arise which can only be successfully coped with by the man 


who has an understanding of the things we have just set 

The action of A. C. is usually expressed by diagram, sim- 
ilar to that shown in Fig. 4, the meaning of the various 
details of which we will endeavor to make clear. 

It is quite essential that the projectionist learn to "read" 
such diagrams, because in the study of the details of his pro- 
fession he will be constantly confronted with them. 

In Fig. 4 the straight, horizontal line represents time, as 
to its length, and zero pressure or voltage, or no voltage at 
all, with relation to the triangles above and below it. Put in 
another way, the horizontal line represents the point at 
which the alternations of the current are completed, and 

Figure 4. 

the voltage and amperage are at zero. Put in still another 
way, it would represent the point at which the sides of coil 
sides A — B, Fig 1, stand one above the other, hence cutting 
no lines of magnetic force and generating no E. M. F. 

From to 1 this line represents the time consumed by the 
current in making one alternation. Triangle A above the 
line represents the voltage and amperage action during one 
alternation. At the left of the vertical line the figures repre- 
sent voltage. 

Consider line B of triangle A. As the armature coil begins 
to cut lines of force, the voltage (and amperage, of course) 


begins to rise, but time is required to accomplish this. If 
the current be 60 cycle it will require l/240th of a second to 
reach the maximum pressure of 110 volts. Line B in its 
length represents the gradual rise of voltage. In its slope 
to the right it represents a lapse of time equal to l/240th of 
a second. It will also require l/240th of a second for the volt- 
age to sink to zero again, as the armature coil gradually 
passes to the position where for an infinitesimal fraction of 
time it cuts no lines of magnetic force, which point is repre- 
sented by the point where the right hand side of triangle A 
crosses the horizontal line at 1. It therefore follows that 
from to 1 on the horizontal line represents l/120th of a 
second, or one complete alternation — the time the current 
has been flowing in one direction. 

The armature coil now enters the magnetic field from the 
opposite direction, as is explained in the text accompanying 
Figs. 1 and 2, whereupon the current begins to flow in the 
opposite direction and the whole action is repeated, as per 
triangle C below the line O, triangles A and C representing 
one complete cycle in course of which l/60th of a second has 
lapsed, and the voltage and amperage have twice risen to 
maximum and twice dropped back to zero. If the current 
were 25 cycle, then from to 1 would represent the lapse of 
l/50th of a second and from to 2 the lapse of l/25th of a 

You will therefore see that in reading diagrams of this sort, 
from right to left means time, and the distance of the triangu- 
lar or curved lines from the horizontal represents E. M. F., 
or in other words, voltage and amperage. 

When studying diagrams of this character it must be re- 
membered that, while the action is almost inconceivably rapid, 
still when plain, single-phase A. C. is under consideration 
twice during each cycle there is absolutely no voltage or am- 
perage on or in the lines. 

It is a bit difficult for the mind of the novice to grasp this 
fact, but it nevertheless is quite true. 

"But," the student inquires, "if there is no volt2ge or 
amperage, how is it that the light from A. C. is continuous ?" 

The answer is that the light from single-phase A. C. is NOT 
continuous in brilliancy. Light is produced by either heating 
carbon or an incandescent lamp filament to the point of in- 
candescence — white hot — and although the brilliancy of the 
carbon or filament fluctuates with each alternation of the 
current, the action is so very rapid that the eye cannot follow 



or even detect it, except in the case of very low cycle current. 
The brilliancy of the light from 60-cycle current is not con- 
tinuous, but to the eye it appears to be so because the eye 
functions too slowly to perceive action of such tremendous 

current may be single-phase, two-phase or three-phase. 
Suppose we have two generators producing precisely the same 
frequency (same number of cycles per second), and that their 
armatures are coupled together by means of a chain belt in 
such way that when the current flow of one is at zero the 
voltage of the other is at maximum. As a result we would 
have "two-phase" current. The voltage of such a circuit is 
never at zero, because when the voltage of one generator is 
at zero in the course of the alternations, the voltage of the 
other is at maximum. The action of two-phase current is 
represented by diagram A, Fig. 5. 

Figure 5. 

If we then couple a third generator to the other two, in 
such manner that the rise and fall of voltage caused by cur- 
rent alternations, as is shown in diagram B, Fig. 5, we shall 
have "three-phase" current. 

Two-phase current ordinarily is transmitted by two entirely 
separate circuits of two wires each. Its advantage lies in the 
fact that whereas single-phase current acts intermittently on 
the armature of a motor, much as does the piston of a single 
engine on its load, two-phase acts the same as does a double 
engine, giving a steady pull to the motor armature. 

Three-phase current requires only three wires for its dis- 
tribution. It is the ideal system for transmitting electric 
energy through any distance for power purposes. It gives 
a practically steady pull on motor armatures. 

For study of these matters in greater detail, we would 
recommend Hawkins' Electrical Guide No. 4, page 997 to 



JT is desirable that the projectionist know the meaning 
of certain terms used in connection with his work, hence 
we append a somewhat extended list of definitions, making 
no pretense that it is complete. It is merely designed to 
define those terms with which the projectionist is likely to 
come more or less frequently into contact. Hawkins' Elec- 
trical Dictionary of electrical terms contains more than 500 
pages of definitions. It is an excellent work for such as 
have need for so complete a list. 

ABSORPTION OF LIGHT.— The retaining or absorption 
by a substance, as a lens, of a portion of the light falling 
upon its surface or passing through it. The energy of the 
light thus retained ordinarily is transformed into heat, though 
in some instances its energy is partly absorbed in the work- 
ing of chemical change. The absorption by good quality 
glass is about one per cent per inch of distance traversed by 
the light. 

ACETONE. — A liquid obtained as a by-product in the dis- 
tillation of wood alcohol. It forms the base of some film 
cement formulas. 

ACTINIC RAY. — A ray of light, or of invisible radiant en- 
ergy which can induce chemical action. The violet and ultra 
violet rays are the most powerfully actinic of any of the 
entire spectrum. 

ADHESIVE TAPE.— See insulating tape. 

A. H. — An abbreviation for ampere hour. 

AIR GAP. — Electrically it means a gap or opening in an 
electric circuit which is occupied by air only, as the gap in 
a gas engine spark plug. 

ALIVE. — A term used to describe the condition of a wire 
or other thing when charged with E. M. F. 

AMMETER. — An instrument for measuring current flow in 
amperes,. It is also known as the "Ampere Meter." It is 
the commercial form of the galvanometer. 

AMP. — The most commonly used abbreviation for ampere. 

AMPERE. — The unit of electrical current flow. See page 


AMPERAGE. — The strength of current flow measured in 

AMPERE-HOUR. — One may draw a certain quantity of 
water, say a gallon, from a hydrant in one minute or in ten 
minutes, but regardless of the time consumed in drawing the 
water, it is still one gallon, no more and no less. The same 
holds true in dealing with electric current. A certain given 
quantity may be used in one minute, or in ten minutes. The 
current flowing in any circuit is the relation of the quantity 
flowing to the time during which it flows. If one ampere of 
current flows for a period of one hour, then one ampere hour 
of energy has been consumed; also a flow of two amperes for 
one half hour would be one ampere hour, as would also a flow 
of four amperes for a period of fifteen minutes, or a flow of 
y 2 ampere for a period of two hours. Amperes x hours= 
ampere hours. 

AMPERE TURN. — A unit of magneto-motive force equal 
to the force resulting from the effect of one ampere passing 
around a single coil of wire. 

ANGLE OF INCIDENCE.— See page 222. 

ANGLE OF PROJECTION.— The angle the axis of projec- 
tion (which see) mal es with a horizontal line level with the 
center of the screen. 

ANGLE OF REFRACTION.— See page 127. 

ANODE. — As applies to projection, the side electrodes of the 
mercury arc rectifier tube. 

APERTURE, PROJECTOR.— The opening in the aperture 
plate of a motion picture projector, the edges of which mask 
the film photograph and give the projected image its outline 
upon the screen. 

of the standard aperture is .906 of an inch wide by .6795 of an 
inch high, which is equivalent to 29/32 and 87/128 of an inch, 

ARC. — In lighting, an arc is the result of maintaining an 
E. M. F. between carbons which are somewhat separated, but 
between which current flows across an "arc stream" composed 
of the gases generated in the process of volatilization of car- 
bon. The resultant brilliancy comes mostly from the incan- 
descent carbon tips, though the arc stream has some luminosity 
by reason of the fact that it carries particles of incandescent 

ARC VOLTAGE DROP.— The drop in voltage between the 
tips of the carbons of an arc lamp, due to overcoming the 


resistance the current encounters in passing from one carbon 
tip to the other. 

ARMATURE. — In a dynamo or motor a core of metal 
mounted on a shaft, around or upon which is a winding of 
wire, the whole being designed to rotate in a magnetic field, 
cut lines of magnetic force and thus produce electric 
energy, or power. 

ARMATURE COIL.— That portion of the winding of an 
armature which would be traced in following an armature 
winding from one commutator segment to the next. 

AUTOMATIC FIRE SHUTTER.— The shutter covering the 
aperture of a projector when it is at rest, the same being 
raised and held open by the action of a governor, when the 
projection speed is such that danger of igniting the film is 

A. W. G. — The abbreviation of "American Wire Gauge," 
which is also and commonly known as the "Brown & Sharpe" 
wire gauge. It is the standard in the United States and 
Canada. It measures wires from No. 40 (diameter .00314 of 
an inch) to 0000 (diameter .46 of an inch). 

AXIS OF PROJECTION.— A straight line from center of 
film photograph to center of the image on the screen. 

BACK END .OF ARMATURE.— End opposite from com- 

BACK FOCUS.— The distance from film to first surface of 
a projection lens when the picture is in focus on the screen 
and the illuminant sufficiently distant to illuminate the film 
with parallel rays of light, as in the case of a film illuminated 
by the sun. A condition never, of course, met in actual 
practice. See "Working Distance." 

BALANCED ARMATURE.— One which will run without 

BALANCED LOAD. — A load carried by two generators, 
as in the 3-wire system, is said to be "balanced" when each 
generator carries an equal load,, or when the load is equal 
on both "sides" of the system. 

BEAM OF LIGHT.— A bundle of light rays. A pencil or 
line of light of greater area of cross section than a single 

BLOWING A FUSE.— The melting of a fuse, usually due to 
overload, though it may be caused by mechanical heat gen- 
erated by poor electrical contact of the fuse with its contacts. 


BLOWING POINT.— The number of amperes (flow of cur- 
rent) necessary to blow a given fuse is the ''blowing point" 
of that fuse. 

BRUSH. — A device for making electrical contact between 
the rotating commutator, or collecting rings of a generator, 
and the stationary circuit wires. Brushes are made from car- 
bon, copper wires, copper strips and copper gauze, but car- 
bon brushes are most largely used. 

BRUSH LOSS. — Loss, in watts, due to lack of perfect elec- 
trical contact between brush and commutator or collector ring. 
May be greatly increased by dirty brushes, dirty, rough 
commutator or lack of sufficient pressure between brushes 
and commutator. 

BRUSH ROCKER.— The rocker or yoke to which dynamo 
and motor brush holders are attached. Its purpose is to per- 
mit the shifting of the brushes around the commutator to 
the neutral point. 

BUS BARS. — Name commonly applied to the heavy copper 
bars used on switchboards where a large number of circuits 
are to be served. Strictly speaking, this name may only be 
properly applied to power house heavy copper bars connecting 
with the generators. 

BUZZER. — An electric signal which makes a buzzing sound. 

B. X. — A flexible metal tubing for the protection of electric 
wires, much used for interior work. A flexible metal conduit. 

CABLE. — A single copper wire, or strand of such wires, 
heavily insulated and covered with a metal sheath, usually of 
a lead composition. 

CALCIUM LIGHT.— An intense white light produced by the 
incandescence of a spot on a pencil of lime when a mixture 
of oxygen and hydrogen gases is burned in contact there- 
with. Also called "lime light." Used for projection where 
electric current is not available, but is a very unsatisfactory 
illuminant for motion picture projection as compared with 
electric light. 

CALIPERS. — Instruments with which to measure external 
and internal diameters. 

CAM. — A revolving disc fixed to a shaft and designed to 
impart to a second element, with which it is in constant or 
intermittent contact, a variable velocity or motion, or an 
intermittent motion. 

CINEMATOGRAPHER.— The one who does the actual 
photographing in the production of motion pictures. 

CANDLE. — The unit of illumination, as one candle power. 


CANDLE FOOT OR FOOT CANDLE.— A unit of illumina- 
tion, being the light given by a British standard candle at 
one foot distance. It is equal to 10764 candle meters, which 

CANDLE METER.— A unit of illumination, being the il- 
lumination of a standard candle at the distance of one meter. 

CANDLE, STANDARD.— The standard candle by which 
all lights are measured is legally held to be a sperm candle 
consuming 120 grains of wax per hour. In practice standard- 
ized incandescent lamps are more reliable. The standard 
unit of candle power established by the National Bureau of 
Standards at Washington equals 100/80ths of the Hefner unit 
under Reichsanstalt standard condition. 

CARBON ARC. — A voltaic arc occurring between carbon 
points, as in an arc lamp. 

CARBON BRUSHES.— Commutator brushes made from 
carbon, sometimes coated with copper to insure better elec- 
trical contact with holders. 

CARBON ELECTRODES.— The carbons used in an arc 

CARBON JAW. — The jaw of an arc lamp by means of 
which the carbons are gripped and held. 

CARBONS. — The carbon rods or pencils used in an arc 

CARRYING CAPACITY.— Greatest number of amperes an 
electrical conductor can safely carry. 

CARTRIDGE FUSE.— See page 109. 

CATHODE. — In projection the lower, mercury, contact of a 
mercury arc rectifier tube or bulb. 

CELLULOID. — A hard, flexible substance formed by dis- 
solving camphor in alcohol and adding pyroxylin. The re- 
sultant mass is incorporated between rollers. 

CEMENT, FILM. — A cement, or chemical solvent, by means 
of which two pieces of film may be joined or spliced together. 
All film cements are volatile, therefore must be kept tightly 
corked when not in use. 

CEMENT LINED CONDUIT.— Conduit having its interior 
surface coated with cement. 

CENTER LENS.— The lens between the collecting and con- 
verging lenses in a 3-lens condenser combination. 

CHANGE-OVER.— In projection, the act of changing from 
one projector to another without interrupting the continuity 
of action upon the screen. 


CHATTERING BRUSH.— The rattling of a brush on the 
face of a commutator, usually caused by the same being loose 
in holder. 

CHOKING COIL.— (Commonly called "choke coil"). A coil 
of wire wound on an iron core so as to give self-inductance 
with small resistance, used on A. C. to impede the current 
with slight loss in power, also called an "impedance coil" or 
"reactance coil." 


CIRCUIT BREAKER.— A device, somewhat similar to a 
switch, by means of which a dangerous variation in current 
flow will operate electro magnets and open the circuit. Cir- 
cuit breakers are made to operate both for over- and under- 

CIRCULAR MEASURE.— Every circle is divided into 360 
equal parts, called degrees. A degree is l/360th the circum- 
ference of a circle, regardless of the diameter of the circle, 
hence a degree has no set dimension as to its width or length, 
but increases in width or length with every increase of circle 
diameter. Each degree is subdivided into 60 minutes, and 
each minute into 60 seconds. 

CIRCULAR MILL.— The area of a circle, l/1000th of an 
inch in diameter. The square of the diameter (multiplying 
diameter by itself) of any circle, in mills (thousandths of an 
inch) gives its area in C. M. 

C. M. — Abbreviation for circular mill. 

CLOSED CIRCUIT.— A circuit in which continuous contact 
permits a constant flow of current. 

COLLECTING RINGS.— Rings of A. C. generator upon 
which the brushes rest, and from which the current passes 
into the brushes and thence to the outside circuit. 

COLLECTOR LENS.— The lens of the condenser combina- 
tion which is next the light source. 

COMBINATION PROJECTOR.— A motion picture projec- 
tor equipped with a stereopticon attachment. 

COMMUTATOR. — An arrangement of copper commutator 
bars by means of which the alternating current of the arma- 
ture is changed to direct current in the outside circuit. 

COMPOUND WINDING.— A method of winding a dynamo 
or motor field magnet with two sets of coils, one of which 
forms a shunt circuit, the other carrying the entire output of 
the armature, except what flows through the shunt circuit. 


CONDENSER. — In projection, a combination of lenses de- 
signed to collect the diverging rays from the light source, 
and to refract and converge them upon the projector aperture. 

CONDUCTOR.— (a) Any substance which will transmit elec- 
tric current, though the name is ordinarily only applied to 
those having low resistance, (b) A wire or a copper bar 
used to transmit electrical energy. 

CONDUIT. — A metal or armored tubing in which electric 
wires are placed for their protection. 

CONJUGATE. — United in pairs ; yoked together ; coupled. 

CONJUGATE FOCI.— See page 127. 

CONNECTOR. — A device for joining wires electrically in 
such manner that they may be readily released. 

CONSTANT CURRENT DYNAMO.— A dynamo so wound 
that it wiU deliver constant amperage under varying load. 
Such a generator varies its voltage instead of its amperage. 

CONTINUOUS CURRENT.— A non-pulsating current which 
is constant both as to pressure and direction of flow. See 
Direct Current. 

CONVERGING LENS.— The lens of a condenser combination 
which is farthest away from the light source. 

COPPER. — Next to silver the best metallic conductor of 
electricity and of heat known. 

COPPER LOSS.— The loss of energy resulting from re- 
sistance offered to the flow of current through a copper 
wire. See Voltage Drop. 

CORED CARBONS.— Projection carbons having a core 
composed of ground, baked carbon, mixed with a suitable 
binder, usually water glass. 

COVER GLASS. — The glass which covers the photograph 
on a stereopticon slide. 

C. P. — The abbreviation for candle power. 

CRATER OF ARC. — The concave depression produced on 
the tip of the positive carbon of arc lamps by action of the 

CRATER ANGLE.— The angle at which the crater is with 
relation to the axis of the optical train. The most efficient 
angle is 55 degrees. 

CRATER PROJECTOR.— A means for projecting an image 
of the crater. It may be a pin hole in the lamp house door 
in conjunction with a lens, or merely a pin hole, or a pin 
hole, a lens and a reflector to direct the image to any desired 


CRITICAL ANGLE.— The angle of incidence beyond which 
rays of light are no longer refracted into a transparent 
medium, but are reflected from its surface. 

CYCLE. — A series of operations. As applied to A. C, the 
cycle is two complete alternations. 

DIMMER. — An adjustable resistance inserted in an incan- 
descent circuit by the manipulation of which the lights of the 
circuit may be gradually dimmed or brightened. 

DIOPTER. — The unit for expressing the refractive power 
of a lens. It is the power of a lens whose focal length is one 

DIRECT CURRENT (D. C.).— A current constant in direc- 
tion, though not necessarily in value. A direct current con- 
stant both in direction and value is called a continuous cur- 
rent. Direct Current, which, while continuous in direction, 
pulsates as to pressure, is often wrongly called continuous 

DIRECT CURRENT CONVERTER.— A machine for con- 
verting D. C. of one voltage to D. C. of a different voltage. 

DISSOLVE. — The gradual transition or fading of one pro- 
jected image into another. 

DIVERGING BEAM.— A light beam which diverges away 
from its immediate source. 

DOUBLE THROW SWITCH.— A knife switch which may 
be thrown over into either of two sets of contacts. 

D. P. SWITCH.— Abbreviation for double pole switch. 

DEGREE. — A unit of measurement of temperature. 

DEGREE. — The circumference of every circle is divided 
into 360 equal parts called degrees, hence a degree is l/360th 
part of the distance around the circumference of any circle. 
It is, therefore, evident that with every increase or decrease 
in circle diameter the linear measurement of a degree changes, 
insofar as applies to that particular circle. Each degree 
is divided into sixty equal parts, called minutes, and each 
minute is divided into sixty seconds. 

DENSITY OF FIELD.— The quantity of electromagnetic 
lines of force existing in a unit of cross section area of an 
electro-magnetic field. 

crease in candle power of an incandescent lamp which takes 
place after prolonged use. 

winding a field magnet with double coils in such a way that 
each exerts a pull against the other. 


DIFFUSION. — As applied to light, its reflection by a sur- 
face in such a way that it is scattered. See page 222. 

DIRECT COUPLED DYNAMO.— One having its armature 
shaft coupled direct to the shaft of the source of power 
which drives it. 

DIRECT CURRENT DYNAMO.— A dynamo supplying di- 
rect current, which may or may not be continuous current. 

DIRECTOR. — One who directs the actors during the mak- 
ing of a scene or scenes of an entire production. 

DOUBLE POLE SWITCH.— A switch which controls both 
wires of a two-wire circuit, as a two-blade knife switch. 

DOUBLE THROW SWITCH.— A knife switch which may 
be thrown over into either of two sets of contacts, thus con- 
necting its center contacts to either of two entirely different 

DOUSER. — A manually operated shutter in the lamphouse 
or in the condenser cone by means of which the light may 
be intercepted before reaching the spot, or, in the case of a 
stereopticon, the lens. 

D. P. SWITCH. — An abbreviation, meaning double pole 

DROP IN POTENTIAL.— A drop in voltage due to resist- 
ance of the lines. May be due to length of lines or to over- 

DRY CELL. — A battery or primary cell usually made up 
of a jar composed of zinc, which forms one of the elements 
or electrodes. The other element is of carbon, suspended 
so that it cannot come into contact with the zinc. The re- 
maining space is filled with an absorbent substance saturated 
with sal ammoniac. 

E. E. — Abbreviation for Electrical Engineer, a degree con- 
ferred upon students by technical schools when they have 
completed a course in electrical engineering. 

EFFICIENCY. — As applied to motors, generators and trans- 
formers, the ratio of power applied at the input terminals to 
the power available at the output terminals. It is found by 
dividing the watts output by the watts input, thus : If the 
input of a motor be 10 amperes at 110 volts, or (100 x 10) 
1100 watts, and the output be 900 watts, the efficiency of the 
motor would be 900 -*- 1100 = 81 + per cent. 

ELECTRODES. — In arc lighting, the carbons which form 
the terminals of the lamp. 

ELECTRO MOTIVE FORCE.— That force which creates and 
maintains an electric current in, on or through a conductor. 


It is commonly termed voltage. It is measured in volts. It 
is abbreviated E. M. F. 

EQUIVALENT FOCUS.— See page 129. 

EXCHANGE. — A central repository from which film may 
be had, usually on a rental basis. 

EXHAUST FAN.— A fan used to pull or pump air out of 
a room, or other inclosure ; a fan designed to create a 

F. — Abbreviation for Fahrenheit. On its scale 32° repre- 
sents the melting point of ice and 212 the boiling point of water 
at sea level. 

FADE-IN. — The gradual appearance of the picture from 
darkness to full brilliancy. 

FADE-OUT. — Opposite from fade-in, which see. 

FEATURE. — A photoplay to be used as the leading part of 
a theatre bill or program. 

FIELD MAGNETIC. — The space occupied by magnetic lines 
of force. 

FIELD COILS. — The coils of wire wound on the field mag- 
nets of a dynamo. 

FIELD MAGNETS. — In a dynamo or motor the magnets 
forming the magnetic field in which the armature revolves. 

FIELD POLES.— The poles of the magnets in the field of 
which the armature revolves. 

FIELD RHEOSTAT. — An adjustable resistance used to con- 
trol the amount of current flowing in the field coils of a dyna- 
mo or motor, hence an adjustable resistance by means of 
which the strength of the magnetic field is varied. 

FILM. — In projection, a ribbon of celluloid upon which the 
photographs constituting a motion picture are carried. 

FILM CEMENT.— See Cement. 

FILM MENDER OR SPLICER.— A device used to correctly 
join the sprocket holes and clamp the ends of the film to- 
gether when splicing film. 

FIXED RESISTANCE.— A resistance having a given, fixed 
value, as a non-adjustable rheostat. 

FLAMING OF ARC. — In projection a flame emanating from 
the tips of the electrodes of an electric arc under certain con- 
ditions. Its cause may be any one of several things, includ- 
ing impure carbon, carbons working above capacity, high am- 
perage and a too great distance between carbon tips. 

FLAT COMMUTATOR SEGMENT.— A commutator seg- 
ment which has become flat through wear, burning or faulty 


FLATS. — Spots on commutator which have become flat, or 
slightly depressed through wear, or from other cause. 

FLOATING BATTERY.— A storage battery so connected to 
a parallel system that it will be charged by it or to automat- 
ically discharge into it, as required. 

FOCUS. — The point of concentration. The point at which 
light rays meet and form an image after being subjected to 
the action of a lens. 

FOCUSING SCREW.— Thumbscrew by means of which the 
projection lens is moved forward or backward to focus the 
image on the screen. 

FOOT CANDLE.— A unit of illumination; the light of a 
standard candle at a distance of one foot. 

FOOTAGE. — Film length measured in feet. 

FRAME (noun). — A single photograph on a motion picture 

FRAME (verb). — To so adjust the projector framing device 
that the film photograph is in correct register over the 

FRAME LINE. — The line between the top of one image and 
the bottom of the next in a motion picture film. 

FREQUENCY. — The number of double alternations per 
second, commonly referred to as "cycles/' 

FRICTIONAL LOSS. — In any machine the amount of energy 
expended in overcoming the resistance caused by the fric- 
tion of its various parts. 

FRONT END OF ARMATURE.— The commutator end. 

FRONT WALL. — As applied to the projection room, the 
wall next the screen. As applied to the auditorium, the wall 
at the stage or screen end. 

FUSE.— See page 107. 

FUSE ALLOY.— See page 107. 

FUSE BLOCK.— A slab or "block" of insulating material 
carrying one or more fuses. 

FUSE LINKS, OR LINK FUSES.— See page 111. 

GAS STREAM.— The stream of gas between the carbon 
tips of an arc lamp. It is formed by the volatilization of 
carbon and has considerable conductivity as compared with 
the surrounding air. 

GENERATOR.— Same as dynamo, which see. 

GERMAN SILVER.— A metal alloy composed of copper, 
zinc, and nickel in varying proportions. Much used for re- 
sistance wire where a uniform resistance at varying tempera- 
tures is important. 


GLASS. — A substance made by melting together sand or 
silica with lime, potash, soda or lead oxide. By varying the 
proportions of these ingredients different kinds of glass are 
obtained, such as bottle, plate, flint, crown, etc. 

GRAPHITE. — One of three forms in which carbon occurs 
in nature. Also called plumbago. Useful to the projectionist 
as a lubricant for the arc lamp, since it is an excellent lubri- 
cant and is but little affected by high temperature. 

GRID. — As applied to projection, one resistance of a grid 
rheostat. See figure 132, page 416. 

GROUND. — (a) Broadly it is a. term used to designate a 
current carrying connection of such high resistance that it 
is not a short circuit, but which nevertheless enables the 
current to reach opposite polarity without traveling its 
allotted path, (b) An electrical contact of one or both polar- 
ities with earth, (c) An electric contact of one polarity with 
something it is not intended shall be electrified. 

GROUND WIRE. — In projection a wire connecting a pro- 
jector frame with earth. 

HARD SOLDER.— A solder which melts at red heat only. 
May be made from zinc and copper. 

HIGH COMMUTATOR BAR.— A condition where one or 
more bars or segments of a commutator are higher than the 
others. Unless remedied it will work very serious harm to 
the commutator. 

HORIZONTAL CANDLE POWER.— Illuminating power of 
a light source in a horizontal direction. 

HORSE POWER.— One horse-power (h.p.) equals 33,000 
foot-pounds of work per minute. It is the theoretical amount 
of work one strong draft horse is supposed to perform if a 
block and tackle be attached to a weight of 33,000 pounds and 
the tackle be of such proportion that the horse can, by exert- 
ing his full strength, just raise the 33,000 pounds one foot 
while walking outward pulling on the rope for a period of 
one minute. Under these conditions one horse-power has 
been exerted during that minute. That is the theory of the 
thing. One horse-power-hour is the amount of work exerted 
by one horse during one hour, or by 60 horses during one 
minute, or by 3600 horses during one second. In electrics 746 
watts is supposed to represent the raising of 33,000 pounds 
one foot in one minute, or, in other words, one horse power. 
The unit was established as follows : 1 watt is equivalent to 
1 joule per second (the joule is the practical C. G. S. unit of 
electrical energy. One joule is equal to .73734 of a foot- 


pound, or, .00134 h. p. -seconds ; it is the quantity of electric 
energy necessary to raise the potential of one coulomb of elec- 
tricity one volt in pressure) or 60 joules per minute, and 1 
joule is equal to .73734 of a foot-pound, therefore 60 joules = 
60 x .73734 = 44.24 foot-pounds. Now, since one horse-power 
equals 33,000 foot-pounds per minute the electrical equivalent 
would be 33,000 ■+■ 44.24 = 746 watts. 

HOUSE SERVICE WIRES.— The wires connecting the main 
house cutout with the street mains or transformer. 

IMAGE. — In projection optics an image is an image or pic- 
ture of an object (transparent photograph on film or slide) 
formed on a receiving surface called a screen, by light rays 
focused by a lens. 

INCLOSED SWITCH.— A knife switch inclosed in a metal 
housing for the protection of its live parts. 

INDUCTION. — The influence which a mass of iron charged 
with A. C. exercises upon surrounding bodies of metal, without 
having any actual metallic connection therewith. 

INSULATION. — The employment of substances having high 
resistance to confine electric current to the conductor, and 
prevent it escaping to a conductor of opposite polarity. Rub- 
ber, porcelain and glass are examples of high resistance 
insulating materials. 

INSULATING TAPE.— A cloth tape impregnated with an 
insulating compound, usually composed of coal tar and resin 
in proportions of about 30 to 40. The compound causes it to 
be adhesive. It is used to insulate wire splices, etc. 

means of which the intermittent sprocket is operated. 

INTERMITTENT SPROCKET.— The sprocket of a pro- 
jector by means of which the film is given its intermittent 
movement at the aperture. 

KILOGRAM. — A unit of mass in the metric system. It 
equals 2.2064 pounds. 

KILOMETER.— 1,000 meters; 3,280.899 feet; or .62138th of a 

KILOWATT.— One thousand watts, which equals 1.34 horse- 

KILOWATT HOUR.— The use of one kilowatt of electric 
energy for one hour. 

KNIFE SWITCH.— A switch having a movable blade or 
blades, usually of copper, which are hinged at one end and 
make or break contact with parallel spring contact clips at 


the other. The switch blade takes the place of the con- 
ductor between its contact points. 

K. W. — Abbreviation for kilowatt. 

K. W. H. — Abbreviation for kilowatt hour. 

LAMP ANGLE. — The angle at which the projector lamp, as 
a whole, sets with relation to the axis of the projector optical 

LAMPHOUSE. — The metal housing surrounding the light 
source and carrying mount for the condenser lenses. 

LAMPHOUSE VENT PIPE.— The pipe leading from the 
lamphouse to the open air, or to some flue connecting there- 
with, by means of which the heat and gases generated inside 
the lamphouse are removed from the projection room. 

LEADER.— A short length of film attached to the leading 
title of a subject, or to the beginning of a reel of film, in order 
to protect it and to allow of threading into the take-up with- 
out using the film title for the purpose. 

LENS. — (a) A transparent medium, usually glass, having one 
or more curved surfaces, for the purpose of changing the 
direction of rays of light, giving them a direction largely 
determined by the curvature of the lens surface or surfaces, 
(b) A combination of single lenses mounted together so as 
to act as a single (compound) lens. 

LENS JACKET. — The outer part of a projection lens, which 
usually carries the focusing mechanism and holds the inner 
lens tube in which the lenses are mounted. 

LENS PORT.— The opening in the front wall of .the pro- 
jection room through which the beam from the projection lens 

LENS, PROJECTION.— See "Projection Lens." 

LIGHT BEAM.— See "Beam of Light." 

LIGHT RAY. — A thin line of light having no appreciable 
area of cross section. 

LOOP. — In projection, the slack film left between the upper 
sprocket and the top of the gate tension shoes, and between 
the lower end of the gate tension shoes and the lower sprocket, 
in order that the film between the two loops may stop and 
start intermittently while the rest of the film has continuous 

MAGNET. — In the ordinary acceptance of the term, a body 
of iron charged wtih magnetism and generating a magnetic 
circuit or field. A magnet may be a permanent magnet (a 
polarized electro-magnet), in which case the magnetic field 
is always present in considerable strength, or it may be an 


electro-magnet only when "excited" by passing a current of 
electricity over wires wound around it. Magnets, either perma- 
nent or otherwise, may be made more powerful by passing 
an electric current over wires coiled around them. 

MAGNET COIL. — A coil of insulated wire wound around an 
electro-magnet, over which current is passed in order to in- 
crease the density of the magnetic field produced by the 
magnet. Also called "Field Coils." 

MAGNET CORE. — The bar of iron or steel around which 
the magnet coil is wound. 

MAGNETIC DENSITY.— The number of lines of magnetic 
force passing through a magnetic field per unit of cross sec- 
tion area. 

MAGNETIC FIELD. — (a) The space immediately surround- 
ing the poles of a magnet through which the magnetic force 
acts. It is strongest near the surface of the magnet poles, 
decreasing rapidly in strength with distance, finally disappear- 
ing entirely, (b) The space immediately surrounding any 
wire conveying alternating current. 

MAGNETIC FLUX. — The average intensity of a magnetic 
field multiplied by its area. The total strength of a magnetic 

MAINS. — A term variously used, but commonly designating 
the wires of the principal distribution circuits of an electric 

MAINS, STREET. — The wires of the street circuit which 
supplies the house service wires. 

MAT. — In projection, the paper mask used to outline the 
photograph of a stereo slide. 

spherical candle power. 

MELTING POINT. — The temperature at which substances 
begin to melt or fuse. An alloy consisting of one part tin 
and one part lead, melts at from 375 to 460 degrees. Tin melts 
at from 442 to 446 degrees. 

Lead melts at from 608 to 618 degrees 

Silver melts at from 1733 to 1873 

Copper melts at from 1929 to 1996 " 

Steel melts at from 2372 to 2532 " 

Wrought Iron melts at from 2732 to 2912 

MENISCUS. — A lens which is convex en one surface and 
concave on the other. 

METER. — The unit of length in the metric system. Equals 
39.37 inches. 


METER. — An instrument for measuring. 

MICA. — A mineral substance, mined in certain places. It 
is semi-transparent, may be split into very thin sheets and 
has high insulating and heat resisting powers. It is used for 
projection arc lamp insulation. 

MIL.— A unit of length. l/1000th of an inch. 

MIL-FOOT. — A wire one mil in diameter and one foot in 

MIL-FOOT STANDARD— The standard of resistance in 
wires. See page 73. 

MILLIMETER. — Abbreviation mm or m/m. One mm equals 
.03937 of an inch. 

MISFRAME. — In a film a wrongly made splice through which 
a part of one photograph is eliminated. In projection the 
showing of a portion of two pictures on the screen at the 
same time. 

MOTOR, ELECTRIC. — A machine for transposing electrical 
power into mechanical power. 

MOTOR GENERATOR. — In projection, a machine consist- 
ing usually of an A. C. motor direct connected to a D. C. 
generator for the purpose of generating D. C. with power, 
supplied by A. C. The resultant D. C. may be of higher, lower, 
or the same voltage as the A. C. supply, but for projection 
work it is, in modern and efficient machines, supplied at arc 
voltage by a generator wound for constant current. Also 
see "Rotary Converter." Motor generators are also used in 
projection for the purpose of reducing D. C. supply to pro- 
jection arc voltage. 

MOTOR GENERATOR SET.— See "Motor Generator." 

MULTI-PHASE— A term applied to any A. C. system hav- 
ing more than one phase. Polyphase is the term more com- 
monly employed. 

MULTIPLE REEL.— A photoplay several reels in length. 

NAILS.— Nails are used everywhere. The following table 
will be useful: 


3 penny 

4 penny 

5 penny 

6 penny 
8 penny 

10 penny 
12 penny 
18 penny 
20 penny 


No. of Nails to Lb. 





1 2/3-inch 















NATIONAL, ELECTRIC CODE.— A national code of rules 
based on the requirements of the fire underwriters. The 
requirements of the code must be observed as to inside elec- 
tric wiring and other work in order to get insurance on a 
building. Copies of the code may be had by applying to the 
National Board of Fire Underwriters, Electrical Department, 
Room 1100, No. 123 William Street, New York City. It is sent 
free of charge, but we would suggest that you send five cents 
in stamps for postage. It is a 216-page book. 

NEGATIVE. — The opposite to positive. In electrical ap- 
paratus the pole toward which the current is presumed to 

NEGATIVE CARBON.— In a D. C. arc lamp the lower car- 
bon to which the current flows across the arc from the posi- 
tive carbon. 

NEGATIVE FILM.— The film which is exposed to light in 
the camera. The film upon which the original image is im- 
pressed. The film from which positive prints are made. 

NEGATIVE POLE.— Opposite pole to positive. In a dynamo 
or battery the pole to which the current is presumed to return 
from the external circuit. 

NEGATIVE WIRE.— A wire attached to the negative pole. 
A wire having negative potential. 

NEUTRAL WIRE. — In a 3-wire system the center wire or 
conductor is the "neutral/ It is negative to one outside wire 
and positive to the other, when either side is used separately. 

NEUTRAL WIRE AMMETER.— An ammeter connected 
into the neutral wire to determine how nearly the load is in 
balance. It may be attached to any circuit or to the service 
wires of any building, if desired. The amount it records is 
the amount the load is out of balance. 

NICKEL STEEL. — Ordinary soft steel to which a small per- 
centage of nickel has been added. Best results are had with 
about 3.25 per cent, of nickel. 

OBSERVATION PORT.— The opening in the front projec- 
tion room wall through which the projectionist views the 

OHM. — The unit of resistance. See page 53. 

OHM'S LAW. — The law that, considering a uniform flow of 
current in a given circuit, the amperage is equal to the E. M. 
F., in volts, divided by the resistance in ohms. The law is 
expressed by simple formulas. See page 55. 

OIL WELL. — (a) An oil-tight receptacle in which the inter- 
mittent movement of a modern projector is placed so that it 


may work in an oil bath, (b) A cavity under a dynamo or 
motor bearing which contains oil for lubrication of the bear- 
ing. See "Ring Oiling/' 

OPEN CIRCUIT.— A circuit which is not complete as to 
electrical connection. A circuit which has been broken, as by 
the opening of a switch., 

OPTICAL AXIS.— A line passing through the center of a 
lens which is perpendicular to its plane. In a projector optical 
train a line from the center of the light source to the center 
of the front lens of the projection lens, when all elements are 
in proper adjustment. 

OPTICAL TRAIN. — In a projector, the various lenses it em- 
ploys referred to as a whole. 

OUTLET. — A point in ceiling or wall out of which wires 
are led to make connection with lamps, motors, etc. 

OUTLET BOX.— An iron box, usually circular in form, lo- 
cated at an outlet to protect the splices and to serve as an 
anchorage for the circuit conduit. 

OUT OF FOCUS.— When the image is not sharp on the 
screen. See page 146. 

OUTPUT. — The electrical energy delivered by a dynamo. 

OUTSIDE TRANSFORMER.— Transformer by means of 
which the service wires of the theatre are fed. Usually re- 
ferred to as the pole transformer. 

OUTSIDE WIRING.— Wiring attached to the surface— not 

OVERLOAD. — A load greater than a machine is designed to 

OVERLOAD CAPACITY.— The amount of overload an 
electrical device or a machine may carry, either permanently 
or for a stated period, without sustaining permanent injury. 

OVER-SHOOTING.— The carrying of the film too far by 
momentum, due to lack of sufficient tension by the gate ten- 
sion springs. Failure of the film over the aperture to stop 
exactly when the intermittent sprocket stops. 

OZONE. — A gas of a faint bluish tint and a characteristic 
odor. It is produced by passing a current of electricity 
through air, changing the oxygen to ozone. It has purifying 
and sterilizing properties. 

PANEL BOARD. — Name applied to a small distributing 
switchboard, usually located in the wall of a room, auditorium 
or hallway, and controlling several circuits, or perhaps all the 
circuits on a single floor. 


PANEL BOARD FEEDERS.— Circuit wires attached to the 
panel board bus bars. 

PANEL BOARD FUSES.— Fuses controlling the circuits 
controlled by a panel board. 

PARALLEL CONNECTION (also called Multiple Connec- 
tion). — (a) A circuit in which two or more incandescent lamps 
are connected between the two wires of the circuit, (b) The 
connection of the two projection arc lamps in such way that 
both positives are connected to positive and both negatives to 
negative, with but one rheostat for both lamps. Under this 
condition both lamps cannot be burned at one time. When 
the carbons of the idle lamp are brought into contact the 
other arc goes out. (c) As applies to rheostats on a projec- 
tion circuit, a connection made in such a way that the wire 
from the supply connects to one terminal of both rheostats 
and the wire leading to the lamp to both of the other rheo- 
stat terminals. See page 426. 

PERFORATIONS.— Holes punched in film- which engage 
with projector sprocket teeth and give film its movement. 
Commonly called "sprocket holes." 

PERMANENT GROUND.— See page 346. 

PHASE SPLITTER.— A device for producing two currents 
from single phase current which will differ in phase, the 
purpose being to assist in starting a single phase induction 

PHASE SPLITTING. — The process of changing a single 
phase current into polyphase currents. 

PHOSPHOR BRONZE.— An alloy of copper, tin and from 2 
to 5 per cent, of phosphorus. This alloy is superior to pure 
copper in strength, but lacks conductivity as compared to 
copper. It is very durable, and is used for projector wearing 
parts by some manufacturers. 

PLANO. — A term used in connection with lenses. It means 
a flat surface. 

PLANO CONVEX.— A lens which is flat on one side and 
convex on the other. 

PLUMBAGO.— See "Graphite." 

POLARITY.— See page 3. 

POLARITY SWITCH.— A D. P. D T. knife switch so wired 
that throwing it over reverses the polarity of the wires of 
the circuit it controls. See page 350. 

POLYPHASE CURRENTS.— Same as two and three-;:hase 
currents, which see, page 18. Any current having more than 
a single phase. 


PORT. — In projection, an opening in the front wall of the 
projection room. 

POSITIVE. — As applied to photography, a "print" from a 
negative. The films used in projection are positive prints. 

POSITIVE BRUSHES.— The commutator brushes of a dy- 
namo or motor which connect with the positive wire of the 

POSITIVE CARBON.— In a D. C. arc lamp the upper car- 
bon; the carbon to which the positive wire of the circuit is 

POSITIVE PRINT.— Film exposed to light passing through 
a negative. The film used in projection is a "positive" print. 

POSITIVE POLE.— The positive (+) terminal of a genera- 
tor from which the current is assumed to flow out to the exter- 
nal circuit. 

POSITIVE WIRE.— The wire connected to the positive 
pole of an electric generator and charged with positive (+) 
E. M. F. 

POWER. — The rate at which work is done, meaning work 
divided by the time in which it is done. The generally ac- 
cepted unit is the horsepower, which is 33,000 foot pounds a 
minute. See "Foot Pound." 

PRIMARY COIL. — In a transformer, a coil consisting of 
many turns of insulated copper wire wound around one "leg" 
of an iron core, or placed within a laminated iron core. In 
effect it is a powerful choke coil, its practical purpose being 
*o create a magnetic field in order that a secondary current 
may be induced in a secondary coil placed within the mag- 
netic field thus created, the voltage of which latter will be 
dependent upon the relative number of turns of wire in the 
two coils. 

PRIMARY CURRENT.— The current in the primary coil of 
a transformer. 
PROJECTION ANGLE.— See page 255. 
PROJECTION DISTANCE.— Distance from projection lens 

to screen. Commonly referred to as "throw." 

PROJECTION LENS.— The lens combination which forms 

the image upon the screen. The lens of a projector optical 

train corresponding to the objective in a camera. Also termed 

"projection objective." 
PROJECTION SPEED.— The speed at which the film is pro- 

jected, expressed in feet a minute. 


PROJECTION SPEED, NORMAL.— Normal speed of pro- 
jection as fixed by the Society of Motion Picture Engineers is 
60 feet of film a minute. 

PROJECTION SPEED, PROPER.— The proper projection 
speed is a speed exactly equal to the camera speed at which 
any individual scene was taken. 

PROJECTION LAMP.— An arc lamp provided with adjust- 
ments necessary to maintain the light source in correct rela- 
tion to the optical train of the projector. 

PROJECTIONIST. — A person who makes the projection of 
motion pictures -his or her profession, trade or business. 
More particularly the title is applied to ambitious, energetic 
men of recognized ability in both practical projection and in 
technical knowledge as applied thereto. 

PROJECTOR, MOTION PICTURE.— A combination of a 
light source, its housing, an optical train and a mechanism 
and a supporting base, with the necessary means for adjust- 
ment of the various elements with relation to each other, 
the whole being used for the projection of motion pictures. 

PROJECTOR MOTOR SWITCH.— The switch attached to a 
projector by means of which the circuit operating its driving 
motor is opened or closed. 

PROJECTOR TABLE SWITCH.— The switch attached to 
the projector by means of which the projector light source cir- 
cuit is opened or closed. 

PUSH BUTTON.— A single pole contact switch for low 
voltage circuits which is operated by pushing a button. 

QUICK BREAK SWITCH.— A switch operated by a spring 
in such way that the contact is broken instantaneously. 

QUIET ARC. — An electric arc which is noiseless in opera- 

RACING. — As applies to a motor or dynamo, the accelera- 
tion of speed which occurs when the machine is suddenly 
relieved of its load. 

RADIUS. — A straight line drawn from the center of a circle 
to any part of its circumference. The distance from the cen- 
ter of a circle to its circumference. Half the diameter of a 

RAIN. — Scratches in film which when filled with dirt be- 
come semi-opaque and have the appearance of "rain" in the 
projected picture. 

the time the intermittent sprocket is in movement to the time 
it is at rest during each cycle of the intermittent movement 


action. It is properly expressed in degrees. A 60 degree 
movement would be one in which the driven member is in mo- 
tion during 60 degrees of the revolution of the driving mem- 
ber, hence the intermittent sprocket is in movement during 
that portion of the cycle. It would correspond to a 5 to 1 
movement, in which the total period is divided into 6 equal 
periods, during one of which the intermittent sprocket is in 
movement. Similarly a 6 to 1 movement would correspond to a 
50 (about) degree movement, the total period or cycle being 
divided into 7 periods. 

RATIO OF TRANSFORMATION.— In a transformer it is 
the ratio of the number of turns in the primary coil to the 
number of turns in the secondary which establishes the rela- 
tion of the voltage and current of the secondary to the volt- 
age and current received by the primary. The relation of pri- 
mary and secondary turns is expressed as follows : Primary 
voltage : secondary voltage = primary turns : secondary turns. 
Primary current : secondary current = secondary turns : pri- 
mary turns. If there be more turns of wire in the primary than 
in the secondary the secondary voltage will be reduced and 
the secondary amperage increased, and vice versa. If there 
be 10 times as many turns in the primary coil as in the sec- 
ondary coil, then the ratio will be 10 to 1, and the secondary 
voltage will be only l/10th that of the primary, while the am- 
perage will be 10 times greater. It will thus be seen that, 
allowing for no loss in the transformer, the wattage of pri- 
mary and secondary is always equal. 

REACTANCE COIL.— See "Choking Coil. ,, 
RECEPTACLE.— A wall socket for an incandescent lamp. 
REEL. — The flanged spool upon which film is wound for 
shipping and for use in projection. 

REEL OF FILM. — The footage carried upon a single reel 
built to carry 1,000 feet of film, when the said reel is approxi- 
mately full. 

REFLECTION.— The change of direction of a light ray 
when it meets a non-absorbing surface and is thrown back. 
See P. 572 Optic Projection. 

REFLECTOR TYPE ARC— In motion picture projection an 
arc having horizontal electrodes, a crater facing away from 
the film and a curved mirror which intercepts the light rays 
from the crater, reflecting them back toward the projection 

REFRACTION.— See page 127. 

REFRACTIVE INDEX.— When a ray of light passes 
obliquely from one medium into another of different density, 
the ratio between the lines of the angle of incidence and 
angle of refraction is known as the "index of refraction" 
of the second medium with respect to the first. 


RELIEF PROJECTIONIST.— The projectionist who works 
a short shift to relieve the regular projectionist during meal 
times or for some other purpose. 

REMOTE CONTROL.— The control of apparatus from a 
point some distance removed therefrom, as, for instance, a 
motor generator located in a basement may be started, stopped 
and controlled from the projection room. 

RESIDUAL MAGNETISM.— As applies to a dynamo, the 
magnetism retained by its field magnet when the machine is 
not in operation. 

RESISTANCE. — That property of an electrical conductor 
which opposes the flow of current. Also the term frequently 
applied to a rheostat. 

RESISTANCE COIL.— A coil of resistance wire, such as is 
used in making up the resistance of a wire coil rheostat. 

RESISTANCE LOSSES.— Losses due to the resistance the 
current encounters in opposition to its flow. See "Copper 

RESISTANCE OF ARC— The resistance offered by the floor 
of the positive crater in the arc stream and the tip of the 
negative carbon. 

RESISTANCE WIRE.— Wire composed of special alloy de- 
signed to offer a fixed pre-determined resistance to current 
flow. It is used for various kinds of rheostats. 

RETURN WIRE.— Same as "Negative," which see. 

REVERSIBLE MOTOR.— An electric motor which may be 
run in either direction, as in street car work. 

REWINDER. — A device for transferring film from one reel 
to another. 

REWINDING. — The process of transferring film from one 
reel to another. This process is necessary each time a film is 
projected in order to change the beginning of the film from 
center to outside of film roll. 

RHEOSTAT. — A device consisting of several units of re- 
sistance which are electrically coupled in such wB.y that the 
current must pass through the entire length of each unit in 
order to reach the next. A rheostat may be adjustable, so 
that the current may be forced through the entire series of 
resistance units, or some of them be cut out, at the will of the 
man in charge, or it may be non-adjustable so that the current 
must pass through the entire series of coils or grids. The 
resistance of a rheostat may be made up of coils of resistance 
wire or banks of cast iron resistance grids. 


RING OILING.— A method of oiling machinery bearings. A 
ring of considerably greater diameter than the shaft is hung 
upon it, the lower portion of the ring extending down into a 
reservoir of oil under the bearing, so that the ring being 
revolved by the shaft, oil is carried up by it to the bearing. 

ROCKER ARM. — That part of a dynamo or motor to which 
the brush holders are attached, which may be rocked back 
and forth to shift the position of the brushes around the 
commutator to the point of least sparking. 

ROTARY CONVERTER.— A dynamo for generating both 
direct and alternating current. Remembering that current 
generated in D. C. dynamo armatures is A. C, it will be 
seen that if the armature current be led to collector rings A. 
C. will be obtained. If the machine be run as a D. C. motor 
A. C. may be had at the collector rings, and if run as a syn- 
chronous A. C. motor, direct current may be obtained from 
the commutator. The rotary converter may also be defined 
as a rotary transformer. 

ROTARY FIELD.— The field created by a combination of 
alternating currents differing in phase, so that if an armature 
of suitable winding be rotated therein the field will rotate be- 
cause of induced currents. The action of an induction motor 
depends upon the creation of a rotary field. 

ROTOR. — In a dynamo or motor, the part which revolves. 

R. P. M. — Revolutions per minute. 

R. P. S. — Revolutions per second. 

RUBBER. — As applies to insulation, rubber may be used in 
different ways. In the form of a rather thin, plastic mass it 
may be placed on a wire and vulcanized. It is used for insu- 
lating tape, and as a vulcanite or an ebonite it is used for 
switch handles, insulating tubing, etc. 

R. C. — Rubber Covered. 


SATURATION. — As applies to a magnet, the highest power 
to which the magnetic flux can be raised. The point at which 
the metal will receive no further magnetism. 

SCREEN. — In projection, the surface to which the picture 
(image) is projected. 

SCREEN, METALLIC SURFACE.— A screen surface con- 
sisting for the most part of metallic powder, such as 
aluminum. Tin foil has also been used with some success. 

SCREEN, MIRROR.— A screen consisting of a plate glass 
mirror, the surface of which has been ground to break up the 
regular reflection. 


SCREEN, SEMI-REFLECTING.— A term commonly used 
to describe metallic and other brilliant surface screens, which 
to greater or less extent give both diffuse and regular reflec- 

SCREEN, DIFFUSING.— A screen which has high powers 
of diffusion of light. 

SCREEN, CALSOMINED.— A backing of suitable material, 
usually plaster or cloth, which is coated with calsomine. See 
page 229. 

SCREEN, PAINTED.— A backing of suitable material the 
surface of which is painted, either with a mixture of white 
lead and zinc, or white, zinc, mixed flat. See page 227. 

SCREEN BORDER.— A border of flat black or other dark 
color surrounding the picture, for which it serves as an out- 
line. See page 240. 

SCREEN BRILLIANCY.— The apparent brilliancy of the 
screen surface as viewed from the auditorium; also the degree 
of brilliancy per unit of area of tfye screen surface as shown 
by photometer measurements. 

SCREEN SETTING.— The immediate surrounding of the 

SECONDARY COIL.— In a transformer, a coil of insulated 
wire in which the secondary current is induced. See "Primary 

SECONDARY CURRENT.— The current induced in the 
secondary coil of a transformer. The current delivered to 
a projection lamp by an Economizer, Inductor or Compens- 

SECONDARY E. M. F.— The voltage of the current induced 
in the secondary coil of a transformer. 

SELF CONTAINED. — A term employed to describe a ma- 
chine the essential parts of which are all contained in one 
frame, or in one foundation. 

SELF OILING BEARINGS.— Machine bearings which are 
oiled automatically by the operation of the machine itself, 
the oil usually being contained in an oil well or reservoir 
located beneath the bearing, from which it is delivered to 
the bearing by suitable means. See "Ring Oiling." 

SERIES. — As applies to electrical machines, lamps or de- 
vices, a connection in such way that the current must pass 
through two or more of them in succession in its passage 
from positive to negative. 

SERIES WOUND DYNAMO.--A dynamo in which the field 
magnets are wound with a few turns of heavy wire which is in 


series with the armature and the external circuit, so that the 
entire output of the armature must pass over the field coils. 

SERIES-SHUNT WOUND DYNAMO.— A dynamo the field 
magnets of which carry both shunt coils and series winding. 

SERIES-PARALLEL CONNECTION.— A circuit containing 
groups of current-using devices connected in parallel (mul- 
tiple) with each other, the groups themselves being connected 
in series. A series multiple connection. 

SERIES WOUND MOTOR.— A motor wound the same as 
a series wound dynamo, insofar as concerns its field coils. 

SERVICE WIRES.— Wires leading into the consumer's 
premises from the street mains. 

SHELLAC. — A resinous varnish the liquid component of 
which is alcohol. It is used as an insulator in certain classes of 
work. It may be thinned with wood or denatured alcohol. 

SHORT CIRCUIT.— Commonly termed a "Short." In the 
common acceptance of the term a fault in an electric circuit or 
apparatus, usually due to defective insulation, by means of 
which the current follows a low resistance by-path to a con- 
ductor of opposite polarity, and either inflicts damage, or is 
wasted in so doing. 

SHUNT. — In an electric circuit a branch conductor joining 
the main circuit at two points, forming a parallel path, so that 
the current is divided, a portion passing through the main cir- 
cuit and a part through the branch. 

SHUNT CIRCUIT.— Same as "Shunt," which see. 

SHUNT COIL. — A coil joined in shunt to the main circuit, 
as the field magnet shunt coil of a dynamo or motor. 

SHUNT WOUND DYNAMO.— A generator having its field 
coil connecting in shunt with the main circuit. 

SHUNT WOUND MOTOR.— A motor having its field coil 
connected in shunt with the main armature circuit. 

"SIDE" OF 3-WIRE CIRCUIT.— In a 3-wire system the 
neutral wire and one (either) outside wire forms one "side" 
of the system. 

SINGLE POLE SWITCH.— A switch which controls only 
one of the wires of a circuit. A knife switch having only one 

SINGLE THROW SWITCH.— A knife switch which makes 
and breaks on one set of contacts only. 

SOLID CARBONS.— Carbons having no "core." Carbons 
having a presumably uniform density throughout. 


SPEED INDICATOR.— As applied to projection, a device 
designed to indicate the speed of projection on a direct reading 


SPHERICAL CANDLE POWER.— The candle power of a 
light source measured in every direction. 

SPLICING. — Joining two sections of a film or wire together. 

SPLIT PHASE. — A single phase current divided into poly- 
phase by means of a phase splitting device. See "Phase 

SPROCKET. — A revolving toothed roller or wheel by means 
of which movement of film through projector is caused and 

SPLIT PHASE MOTOR.— A single phase induction motor 
provided with a phase splitting device for starting. 

STANDARD CANDLE.— A sperm candle so made that it 
consumes 120 grains of wax an hour. 

STANDARD FILM.— See page 130. 

STAR. — As applies to projection, the member of a star and 
cam type of intermittent movement to which movement is 
imparted by the actuating cam. The part of an intermittent 
movement of the star and cam type which is attached to the 
intermittent sprocket shaft. 

STEEL. — A compound of iron, about 3 per cent, of carbon 
and usually small quantities of silicon and manganese. It is 
the carbon which causes the metal to harden when cooled 
suddenly from red heat, and to soften again when cooled 

STEP-DOWN TRANSFORMER.— A transformer which re- 
duces the primary voltage and increases the amperage in 

STEP-UP TRANSFORMER.— A transformer which de- 
livers higher secondary voltage than the impressed primary 
voltage, decreasing the amperage in proportion. 

STEREO. — Contraction of the word Stereopticon. 

STEREOPTICON— A light source and optical train, to- 
gether with the necessary housing and mechanism for holding 
and adjusting the lenses, for the projection of still pictures 
(transparencies) to a screen. 

STOPPING DOWN.— In projection, the act of reducing the 
diameter of the free opening of a lens. 

STORAGE BATTERY.— A form of battery which, when 
subjected to the action of an electric current undergoes cer- 
tain chemical changes, enabling it to produce electric power 


in proportion to the energy consumed in producing the 
change. It is erroneously called a "storage" battery. As a 
matter of fact there is no actual electricity in a "storage" 
battery, but instead there is an ability to produce or generate 

STRANDED WIRE. — A wire composed of several smaller 
wires, usually twisted together. 

made up of a large number of very small wires (usually No. 
30 or 31 B & S) in order to obtain flexibility, covered with as- 
bestos insulation so that it may be used in places where the 
heat is too great to allow of the use of ordinary insulation. 

STRIKING AN ARC— The act. of bringing the carbons of 
an arc lamp into contact and separating them again in order 
to obtain an electric arc. 

A. S. W. G. — Abbreviation for American Standard Wire 
Gauge, commonly known as the "B and S" wire gauge. 

SYNCHRONISM.— A relation as to time of the pressure 
waves of two or more alternating currents when combined 
in an electric distribution system. See Fig. 5. 

SYNCHRONIZING.— So regulating the operation of two 
A. C. generators that they will be identical and simultaneous 
both in phase and frequency. 

SYNCHRONOUS MOTOR.— An A. C. motor which must be 
brought up to speed and into step with the phase of the gen- 
erator before being connected direct to the circuit. Such a 
motor is to all intents and purposes an ordinary A. C. genera- 
tor run as a motor. 

TAKEUP. — A device by means of which the film is wound 
upon a reel as fast as it comes from the projector mechan- 

TAKEUP PULL.— The pull exerted by the takeup tension. 
It must not exceed 15 ounces at the periphery of a 10-inch 
reel, or 16 ounces on an 11-inch reel. 

TAKEUP TENSION.— See "b" under "Tension." 

TAPED JOINT. — A wire joint or splice, wrapped with in- 
sulating tape. 

TAPE, INSULATING.— See "Insulating Tape." 

TENSION. — As applied to projection (a) the pressure ex- 
erted by the tension springs, either through the tension shoes 
or direct, upon the film at the aperture, (b) The tension ex- 
erted by a spring upon two friction discs for the purpose of 
revolving the film reel in the lower magazine, providing slip- 
page so that the reel, although driven by a power having 


steady speed, may revolve at differential speed, gradually 
slowing up as the film roll increases in size. 

TENSION SHOES.— Metal bars upon which the tension 
springs bear which themselves bear directly on the film and 
provide braking friction to stop film over aperture. 

TENSION SPRINGS.— The springs which provide braking 
action to stop the film over the aperture at the completion 
of the intermittent movement. 

THREE-PHASE.— See page 18. 

THREE-WIRE CIRCUIT.— A circuit in which all three 
wires O'f a three-wire system are used. 

THROW.— See "Projection Distance." 

THREE-WIRE SYSTEM.— See page 85. 

THUMB MARK.— A mark on the lower left hand corner of 
a stereo slide when the slide is held so as to be read against 
the light. ■ Thumb mark is on the face (cover glass) side of 
the slide. 

TORQUE. — The pulling force which tends to rotate, as the 
rotating of a motor armature. 

TRAILER. — A short length of opaque film attached to the 
end of a reel of film so that projection may proceed up to the 
end of the film proper without showing white light upon the 

TRANSFORMER.— A device for transforming A. C. of high 
voltage to a lower voltage and increased amperage, or vice 
versa. See page 596. 

TRAVELING ARC. — An unsteady arc, particularly one in 
which the point of highest illumination in the crater moves 
about, usually due to faulty carbons. 

— A knife switch with three blades, but which makes and 
breaks on one set of contacts only. 

TWO-PHASE CURRENT.— See page 18. 

TWO-PHASE MOTOR.— An induction motor which, in- 
stead of having a single field winding, has two separate wind- 
ings, each taking current from a single phase circuit of the 
same frequency, but differing in phase by one-quarter of a 
period. A motor made to operate on two-phase lines. 

TWO-PHASE SYSTEM.— A system supplied with two al- 
ternating currents of the same frequency, but differing in 
phase by a quarter of a period. It may be supplied by two 
separate two-wire circuits, or by a three-wire system in 
which one wire is common to the two currents. 


TWO-WIRE SYSTEM.— A system in which only two wires 
are employed. 

ULTRA VIOLET RAYS.— Rays of light existing beyond the 
violet light of the visible spectrum. These rays have a viDra- 
tion in excess of eight hundred billion per second. 

UNBALANCED LOAD.— As applies to the 3-wire system, a 
condition in which more current is consumed on one side of 
the system than on the other, which has the effect of com- 
pelling one generator to carry a heavier load than the other. 

UNDERWRITERS' RULES.— See "National Electric Code. ,, 

USEFUL LIFE. — A term applied to many things, but in 
electrics particularly to incandescent lamps, which deteriorate 
with age. It is the time an incandescent lamp will burn before 
its output of light has decreased more than 20 per cent. 
When a lamp has fallen below 80 per cent, of its rated c. p. 
it is the part of true economy to replace it with a new one. 

VOLATILIZATION. — In projection, the transforming of 
carbon into vapor through heat. 

VOLT. — Unit of electrical pressure. See page 50. 

VOLTAGE DROP.— The drop in voltage due to the resis- 
tance of the conductors. Voltage drop exists in every circuit. 
See "Copper Loss." 

VOLT-AMMETER. — An instrument for measuring current 
consumption in watts. 

VOLTMETER. — An instrument of high resistance for meas- 
uring electrical pressure. 

WATER GLASS.— Soluble glass. Used as a binder for 
carbon cores. It is the residue of water glass which forms 
the white, light weight ash with which the interior of the 
lamphouse becomes coated. 

WATT. — The practical unit of electrical power. See page 53. 

WATT HOUR. — The energy consumed when one watt of 
electrical energy has been used for one hour. 

WATT-HOUR METER.— A meter used for measuring elec- 
trical consumption in watt hours. 

WATTS PER CANDLE POWER.— The specific consump- 
tion of an electric lamp is its watt consumption per mean 
spherical candle power. In connection with incandescent 
lamps, the term "watts per candle power" is usea commercially 
to denote the watts consumed per mean horizontal c. p. 


WIRE GAUGE. — A gauge for measuring round wires ac- 
cording to an arbitrary standard. See page 78. 


WORKING APERTURE.— In projection, that portion of 
the aperture of a lens which is actually in use in the sense that 
it is contributing to the improvement of the finished screen 

WORKING DISTANCE.— The distance from film to first 
surface of a projection lens when it is adjusted to actual 
working conditions. See "Back Focus." 



Electrical Terms and Their 

THERE are a few electrical terms with which the pro- 
jectionist comes into quite intimate contact in his daily 
work. It is essential that he have a thorough under- 
standing of what they actually represent to the end that he 
be able to use them intelligently in the various calculations 
arising from time to time in his work. 

POLARITY. — Polarity and Potential mean the same thing. 
When wires are attached to opposite terminals of the same 
dynamo there is present in these wires an electrical condition 
which enables them to perform work. Properly connect them 
to an electric motor and the energy in or on these wires will 
cause its armature to rotate and exert a pull and thus pro- 
duce power. 

Attached to a lamp the energy in or on these wires will 
cause the filament or the carbons thereof to become white 
hot, and thus produce light. 

This electrical condition is termed "polarity" or "potential." 
It is, or it represents the electrical affinity which the posi- 
tive wire of an electric circuit has for the negative wire of 
the same circuit, or the negative wire of any circuit attached 
to the same dynamo. It represents the inclination of the 
electric impulse to "flow" from positive to negative, which 
same is termed "current flow." See Page 3. 

When wires are charged with direct current one wire is con- 
tinuously positive and the other negative. 

When wires are charged with alternating current each wire 
of the circuit is alternately positive and negative many times 
every second. If it be 60 cycle current, then each wire is 
.positive 60 times and each wire negative 60 times per second. 

Electric current is usually treated as having both pressure 
and volume. In its action as relates to these attributes, as 
well as regards the item of friction, electricity is very similar 


to, and may be compared with water, but it must be remem- 
bered that the similarity exists in similarity of action only. 

Water may be subject to physical examination. We can 
feel it, watch its action and weigh it. On the other hand, 
electricity is an absolutely impalpable substance — if we may 
even call it a substance. It apparently is without weight. 
We cannot see it, except in the form of light, which is not 
the current itself but a product of its action. We cannot feel 
it, except in the form of a "shock" occasioned by the current 
passing through the tissues of the body. 

Voltage corresponds in effect, or in its action, to the pres- 
sure of water in a pipe, or to the pressure of steam in a boil- 
er. A dry battery, such as is used for electric bells, has a 
pressure of approximately one volt. It imparts that pressure 
to wires connected to its terminals, so that if you attach two 
wires to such a battery, they will, at any portion of their 
length, have an electrical pressure of one volt. If you con- 
nect the zinc of a second battery with the carbon of the first 
battery by means of a short piece of wire, and then attach 
two other wires to the two remaining binding posts, you will 
have what is known as "series 5 ' connection, and a resultant 
pressure of two volts between the two last named wires. A 
third battery connected in series would raise the pressure to 
three volts, and so on indefinitely. 

Instead of using batteries for producing light and power, 
which would be entirely impractical, we use a machine called 
a dynamo, each one of which is designed and built to produce 
a certain voltage, which may be anywhere from one to 500 
volts D. C, or from one to 6,000 volts A. C. 

Remember that voltage corresponds to pressure, and is 
similar in its action to pressure in a steam boiler, but that 
voltage acts only between the positive and negative wires of 
the dynamo or battery which generated it, and that the posi- 
tive attached to one generator has no affinity or attraction 
to or for the negative attached to another dynamo, or for the 
ground, except as it offers a path to the negative of the gen- 
erator to which the positive is attached. Get this fact firmly 
fixed in your mind. Ninety-nine out of every hundred non- 
electricians believe current generated by a dynamo seeks to 
escape into the ground. This is not so, except insofar as the 
ground may offer a path of electrical conductivity between 
two wires of opposite polarity. See page 6. 

AMPERE is the term used to denote quantity. It repre- 
sents the volume of current flowing through, or along a wire, 


just as gallons or barrels represent the quantity of water 
flowing through a pipe, or cubic inches the volume of steam 
flowing. As a matter of fact we do not actually know that 
anything flows in or along the wire of an electric circuit. 
Eminent theoretical electricians say there is an actual flow; 
other equally eminent theoretical electricians say there is 
not, but that what we consider as current flow is really a 
"molecular bombardment." With these highly technical 
questions, however, we have no concern. For our purpose it 
is sufficient to say that the current flows along the wires, ex- 
actly as water flows through a pipe. 

HOW WORK IS ACCOMPLISHED.— The work performed 
is accomplished by voltage, or pressure, working through am- 
perage, or volume. When a water wheel is turned by water it 
is not the water but the pressure which is consumed. True, 
the work is sometimes done by falling water, in which case the 
weight of the water amounts to and is the same as pressure, 
and in that case it is the pressure produced by gravity or 
weight which does the work. It, therefore, follows that the 
higher the pressure or voltage the greater the amount of 
work a given current volume or number of amperes can 

For instance, if we supply an engine with steam at fifty 
pounds pressure, there will be a certain, definite number of 
cubic inches of steam used at each stroke of the piston, and 
a certain definite number of foot-pounds of work will be ac- 
complished. But if, without changing anything else, we raise 
the steam pressure to one hundred pounds, the engine, while 
still using precisely the same number of cubic inches of steam 
at each stroke of the piston, will perform twice as many 
foot-pounds of work. In this case, while the volume of steam 
used remains unaltered, the pressure is doubled, in both cases 
the pressure is entirely consumed, but the volume of steam 
remains and is not consumed. The same power would be 
produced by the lower pressure if the area of the piston be 
doubled, since twice as much pressure would be made avail- 
able through increase of volume ; but again it would be pres- 
sure, not volume, which would be consumed. 

It is the same with electric current. Ten amperes of cur- 
rent at 50 volts represents a certain, definite amount of en- 
ergy. Ten amperes at 100 volts represents just twice as 
much, though it is also true that twenty amperes at 50 volts 
would amount to the same thing in power production. The 
point we seek to make is that amperage or volume is merely 


the vehicle through which pressure or E. M. F. works, and 
that in the production of power in any form it is voltage 
(pressure) and not amperage (volume) which is consumed. 

In a steam engine, with the steam at given pressure we 
may increase the power of the engine by either increasing 
the area of the piston or the length of its strokes or by in- 
creasing the pressure of the steam. In a water motor we may 
increase the capacity to do work either by increasing the 
size of the motor or the pressure of the water. The same 
thing holds true with electricity. We may increase its ca- 
pacity to do work either by increasing the volume of current 
(amperage) or by increasing the voltage. To perform a given 
amount of work with a low pressure (voltage) a large volume 
(amperage) is necessary, but if the voltage be high the same 
amount of work can be performed with much less volume of 
current. The horse power of work performed by electric 
current is represented by the voltage times the amperes di- 
vided by 746.H.P. = Volts x Amperes -*- 746. 

OHM. — In passing through a pipe water encounters re- 
sistance by reason of the rough sides of the pipe, as well as 
by reason of the internal resistance of the water itself. This 
resistance tends to retard the flow. Precisely the same is 
true with electricity. In passing through a wire electric 
current encounters resistance, and this resistance tends to re- 
tard the flow of current. It is measured in ohms, the defini- 
tion of which is given on page 35. 

The effect of resistance is to produce heat. In other words 
power consumed in overcoming resistance is transformed into 
heat. In a water pipe the resistance increases as the volume 
of water passing through a pipe of given diameter is in- 
creased, or as the diameter of the pipe is made smaller with 
relation to the volume of water flowing. The same thing is 
true of current. Having a wire of given diameter its resist- 
ance increases as the current flow becomes greater, and de- 
creases as the current flow becomes less, or, having a given 
current flow, the resistance decreases as the diameter of the 
wire is made greater, or its length is decreased. 

WATT.— Watt is the unit used to measure the amount of 
electrical energy expended — the amount of work actually per- 
formed. It is found by mutiplying the voltage by the number 
of amperes, and is transformed into horsepower by dividing 
that result by 746, since 746 watts is equal to one electrical 
horsepower. For example: If we have 10 amperes at 110 
volts the amount of energy expended would be equal to 110 x 


10 = 1100 watts, which, divided by 746, equals 1.47 h. p. If, 
on the other hand, we use 110 amperes at 10 volts, the result in 
power would be the same. But if we use 10 amperes at 
10,000 volts we then would have 10,000 x 10 = 100,000 watts, 
which, divided by 746 equals 134 h. p. 


projection rooms not equipped with a reliable voltmeter and 
ammeter it will be difficult for the projectionist to- make in- 
telligent calculations, since in order to find a desired quan- 
tity, be it voltage, amperes or ohms, he must know the value 
of the other two. In order to accurately calculate the num- 
ber of amperes flowing in a circuit it is necessary to know 
exactly the number of ohms resistance the circuit offers (in- 
cluding wires, appliances, etc.) and the exact voltage. 

It is, however, highly desirable that the projectionist under- 
stand how to make electrical calculations, at least insofar as 
applies to his projection circuit, since under some circum- 
stances such knowledge will be necessary to intelligent work. 

The projectionist must firmly fix in his mind the fact that 
where the projection circuit is concerned the resistance does 
not lie wholly in the rheostat, or whatever takes its place. 
The wires, carbon-arms and carbons usually offer compara- 
tively slight resistance, but a very considerable portion of 
the total resistance of a projection circuit is in the arc itself. 
Under usual conditions the resistance of the wires, carbon- 
arms and carbons may, for the purposes of calculation, be 
neglected, but unless the resistance of the arc itself be taken 
into consideration, very serious error will result. 

When making electrical calculations it is customary, for 
the sake of brevity, to use the letters E, C, and R, in which 
E stands for "electromotive force," which is merely another 
name for voltage ; hence E stands for voltage ; C stands for 
current flow, meaning amperes ; hence C stands for amperes ; 
R stands for resistance in ohms; hence R stands for ohms. 

The projectionist should also remember that in a common 
fraction the horizontal line always means "divided by," thus 
1/2 really means 1 -*- 2. To divide 1 by 2 we put down the 1, 
followed by a period, called a "decimal point" and then add 
ciphers thus: 1.00. We now have 1.00, with a decimal point 
between the one and the two O's. In dividing 1 by 2 we only 
need one cipher, thus 1.0 and 1.0 -*- 2 = .5, which is exactly 
the same thing as 5/10, or 1/2. The rule is to count the fig- 
ures at ciphers to the right of the decimal point in the num- 
ber being divided, and then, beginning at the last figure of the 


result, count an equal number, and place the decimal to the 
left of the last figure counted. If there are not enough fig- 
ures in the result to do this, add enough ciphers on the left. 
When dealing with formulas, E/C means that the quantity 
represented by E is to be divided by the quantity represented 
by C, E being the voltage and C amperes. If there be two or 
more quantities above or below the line, with no sign between 
them, it means that they are to be multiplied together, thus : 

— means that E (volts) is to be divided by C (amperes) mul- 


tiplied by R (ohms). means that after 15 has been sub- 

tracted from the quantity represented by E (volts), the re- 
mainder is to be divided by the quantity represented by C 
(amperes). The student will be greatly benefited if he will 
practice writing out formulas of this kind in letters, after- 
ward substituting quantities in figures and working them 

Ohms law sets forth the fact that the number of amperes 
flowing is equal to the voltage divided by the resistance in 

ohms. We therefore have — = C, or in other words, volts 


divided by ohms equals amperes. It then follows that if — = C, 

C multiplied by R must equal E. It also follows that — = R. 

It works out as follows: We know that the ordinary 110 volt, 
16 c. p., carbon filament incandescent lamp requires approxi- 
mately one-half ampere of current to bring it up to candle 

power. What is its resistance? Using the formula — = R, 

110 volts 

substituting figures, we have = 220, the number 

.5 of an ampere 
of ohms resistance in the filament of the lamp. Again applying 
E 110 

the formula — = C, we have = .5, or y 2 , as the amper- 

R 220 


age 110 volts will force through 220 ohms resistance. All this 
seems simple enough of understanding and application, but to 

make it yet more plain we will consider the formula — = R, 

which means voltage divided by amperes equals ohms. If 
the voltage be 50 and the amperes 10, then E would be 50, C 
would be 10, and R would be 50 -*- 10 = 5. If the voltage 
were 110 and the amperage 5, then E would be 110, C would 
be 5, and R would be 110 -*- 5 = 22 ohms. 

When, however, we come to consider the projection arc cir- 
cuit a new element enters in the shape of the resistance of 
the arc itself, and if we propose to be absolutely accurate we 
must consider also the resistance of the carbon arms, wires, 
etc. That degree of refinement, however, is seldom, if ever, 
necessary in a projection circuit calculation. 

In leaping the gap between the carbon tips of the arc lamp 
the current encounters high resistance. In overcoming re- 
sistance voltage is consumed, as will be more thoroughly set 
forth and explained under "The Light Source." In other 
words, when current flow is opposed by resistance, and that 
resistance is overcome, there is a consequent drop in pres- 
sure or voltage; pressure having been consumed in the process. 

For reasons not necessary to enter into at this time, the 
D. C. arc, for a given amperage, is longer than the A. C. arc. 
It therefore follows that its resistance will be higher. The 
accepted theory is that all voltage is consumed at the arc. 
Whether or not this is true is a highly technical question, 
which it would be unprofitable to discuss in this book. 

We shall accept the theory as stated. It then follows that 
the rheostat, or whatever takes its place, must reduce the volt- 
age to just that pressure which the resistance of the arc will 
consume when the arc is burning normally. 

ARC VOLTAGE CONSTANTS.— Under varying conditions 
projection arcs will operate at their best at different voltage 
drop. Tables on pages 400 and 400^ are as reliable as any 
though various carbon combinations would undoubtedly alter 
the results as set forth therein in considerable degree. 

The figures in tables 21 and 22 are NOT given as an accurate, 
unvarying factor. They are designed as a fairly ac- 
curate guide only. They may be safely used in figuring neces- 
sary resistance for a temporary set-up. 

And now let us illustrate the method of applying this 
knowledge in practice. 


. Taking a 60 ampere arc, for example, what is its resistance? 

Accepting for the sake of simplicity in figuring the constant 

60 volts as approximately correct for a 60 ampere arc, we 

E 60 

then have — = R. Substituting figures we then have — = R, 

C 60 

and 60 -*- 60 = 1 ohm, which is the arc resistance, or the re- 
sistance necessary to consume 60 volts. 

Let us prove this. Suppose the line voltage to be 110. The 
total resistance necessary to allow 60 amperes to pass must 

then equal — = R the voltage divided by amperage; hence 

amperes being 60 and voltage 110, the resistance will be 
110 -s- 60 = 1.833 ohms. We have already seen that the re- 
sistance of the arc is 1 when the arc voltage is 60. Subtract- 
ing the arc voltage from the line voltage (110 — 60) gives 50 
as the drop in voltage there must be across the rheostat. In 
other words, there must be 50 volts of electric energy con- 
sumed, or "used up" in the rheostat, which will appear in the 
form of heat. 

Again applying the formula — = R, we -have 50 -*- 60 = 

.833+ (the + sign meaning that the division could be carried 
further) as the ohmic resistance of the rheostat. If we now 
add the resistance of the arc and the rheostat together (1 
plus .833) we shall have as a result 1.833, which corresponds 
to the total resistance necessary to allow 60 amperes to pass, 
the slight discrepancy as between 1.832 and 1.833 being due 
to the division being only carried to the fourth decimal point. 
If the amperage were more than 60, line voltage remaining 
the same, then the total resistance would be less, since volt- 
age divided by amperes (E -*- C = R) equals resistance, and 
the result obtained by dividing 110 by a number greater than 
60 is a less number of ohms. Conversely, if the amperage be 
less than 60 the total resistance necessary (arc and rheostat) 
would be greater. 

The higher the line voltage the greater must be the resist- 
ance to accomplish a given current flow, as will be seen by 

applying the formula — = R. 



practice we amend the before named formula for general cal- 
culations in such a way as to automatically take care of volt- 
age drop, thus: Suppose we wish to calculate the necessary 
ohmic resistance of a rheostat to pass 60 amperes D. C. from 

110 volt lines. Instead of using the — = R, we amend it thus : 


= R, the "55" being the constant for voltage drop of 

a 60 ampere D. C. arc. Substituting figures we would then 

have = .9166 ohms, and we thus at one operation 

have the result sought. In using this latter formula we would, 
of course, use the voltage drop constant for the amperage 
used in each case. 

A. C. VOLTAGE DROP.— When figuring A. C. projection 
arc voltage drop we must use a different constant, consider- 
ably lower in value. See page 400. 

As an example of the possible actual practical value of 
knowledge, such as is contained in the immediate foregoing, 
suppose you are called upon to take charge of projection in a 
theatre employing a private 125 volt light plant, using 50 am- 
peres at the arc. You find the rheostats in bad shape and 
order new ones. Instead of ordering a 50 ampere, 110 volt 
rheostat you, discarding unintelligent guess-work, apply the 

formula = R, or, better still, measure the actual voltage 

of the arc by disconnecting the voltmeter and touching one of 
the wire attached to its terminals to each carbon of the pro- 
jection lamp, when you have the arc burning normal. Sup- 
pose the voltmeter reads 52. You would then have the for- 
. 125-52 

mula = 1.46 ohms as the necessary resistance of a 

rheostat to deliver exactly 50 amperes, under the conditions 
at your plant. Would you not rather be able to hand the 
manager an order for exactly what you want, and need, than 
an order which will probably result in your need being only 
met approximately? 


RULE O' THUMB. — There is a very simple formula, easy 
of application, which combines the three formulas into one. 
It is called the "Rule of Thumb." It is expressed for general 

use as : 


To use the formula you have but to cover the symbol or 

letter representing the quantity desired, and what remains 

will produce the answer, thus : Suppose we wish to ascertain 

the resistance in ohms. We cover up the "R" in the formula 

and find that we have — remaining, which will give R, the 

desired quantity. In using this formula on projection circuits 
the top letter must be expressed as E minus the arc voltage, 
the same as in the regular formulas. 




THE one thing which enters into all problems of the 
electrician and the projectionist is resistance. It is 
met with in every phase of electrical work and so far 
as the production of light be concerned, it is the very key- 
stone of the structure. 

When an electrical impulse passes through or over a wire 
it encounters resistance, which in its action is very similar 
to the resistance encountered by water in flowing through a 

When water flows through a pipe it encounters resistance 
which will be directly in proportion to the diameter and the 
length of the pipe, the roughness or smoothness of its interior 
surface, and the quantity of water flowing per minute'. To 
some slight extent this resistance is the result of molecular 
friction within the water itself, but for the most part the 
friction is between the water and the walls or sides of the 

In a pipe of given diameter the resistance will increase 
with (1) increase of flow or volume of water, (2) increase of 
the length of the pipe and (3) with increase in roughness of 
the interior walls of the pipe. 

Conversely, resistance will decrease as the flow of water is 
diminished, the length of the pipe decreased or with increas- 
ing smoothness of the walls of the pipe. 

With a given flow of water, resistance will increase as the 
length of the pipe is increased, as the diameter of the pipe 
is made smaller or as the roughness of the walls of the pipe 
increases, or resistance decreases as the pipe diameter is 
increased, the pipe made shorter or as its walls become more 

Pressure is the motive power, either in the case of water 
or electricity, and resistance always consumes pressure, and 
consumes it precisely in proportion to the amount of resist- 
ance encountered. In the third edition of the handbook we 
explained this proposition by means of a diagram which we 
do not believe can be materially improved upon, hence it is 
herewith reproduced as Fig. 6. 



In Fig. 6 we see a large water main, upon the top of which 
is mounted a gauge registering 100 pounds pressure. To 
this main, pipes A B and C are connected. Pipes A and B 
have a half inch interior diameter. Pipe B is short, having 
a length, let us assume, of one foot. The water from it 
spurts out under high pressure, carrying itself almost hori- 
zontally over a considerable distance. Pipe A we will assume 
to have a length of 100 feet (in a drawing of this kind it is 
impossible to draw the pipe lengths in correct proportion and 
still make the details large enough to be understandable, 
therefore we assume pipe B to have a length of one foot and 
pipe A 100 feet, that being about the proportions that would 
give something approaching the results shown). You will 


Figure 6. 

observe two things with relation to pipe A: First, that where- 
as gauge 1 registers almost the same pressure as the gauge 
on the large main, gauge 2 registers very much less, and the 
water from pipe A spurts out with but little force. The rea- 
son for this is as follows : Pipe B is short, and whereas the 
water encounters high resistance because of being forced 
through with great rapidity, still there is not much length 
to the pipe, hence comparatively little of the total pressure 
is consumed in forcing the water through pipe B, even at 
high speed. Therefore it spurts out at the end of the pipe 
with great force. Pipe A, on the other hand, is 100 feet 
long, and the water in flowing through that length of half 
inch pipe would, at the same rate of flow, encounter 100 times 
the resistance offered by pipe B. Of course, the actual re- 
sistance encountered is not 100 times as great, because as 


pressure is consumed in overcoming the resistance of the long 
pipe the movement of the water is slowed down. In this 
we see the exemplification of the effect of added resistance 
due to increased length of pipe, diameter remaining the same. 

Now let us consider pipe C, which we will assume to have 
a total length of 10 feet of three inch, and two feet of half 
inch pipe. We observe that gauge 3 on pipe C registers es- 
sentially the same as does the gauge on the water main. This 
is because, since only the capacity of the half inch pipe at its 
end can flow, the water in pipe C is moving slowly, hence 
encounters very little resistance. In other words, pipe C is 
working far below its normal capacity. The short pipe at 
its end, however, is working far above its normal capacity, 
but it is short, hence the resistance it offers is comparatively 
slight, and gauge 4 registers but little less than gauge 3 and 
the water-main gauge. 

We could go on at great length, showing the action of re- 
sistance as applied to water, but we think enough has been 
said to make the meaning clear. 

The pressure under which the water might be would not 
affect the result, except that the higher the pressure the 
greater the amount of resistance which can be overcome, and 
vice versa. 

A pipe of given diameter will carry water up to its capacity 
under any pressure sufficient to move the liquid and less than 
that sufficient to burst the pipe. A pipe of given diameter 
will, however, only convey a certain number of gallons of 
water per minute without excessive friction, regardless of 
whether' the pressure be ten pounds or fifty pounds per 
square inch. 

When the point is reached where resistance to flow be- 
comes excessive, the normal capactiy of the pipe is said to 
have been reached. 

It is quite true that we may still force a greatly increased 
volume of water through the pipe, but it can only be done 
at the expense of largely increased power consumption. It is 
a costly proceeding to force a water pipe above its normal 
capacity, and the cost increases very rapidly as excess over 
capacity is increased. 

Where it is necessary to convey a certain volume of water 
per minute, and the pipes are overloaded, the practical meth- 
od of reducing the resistance attendant upon overload is to 
increase the diameter of the pipe until the desired flow is 


obtained with only normal friction loss. Wc therefore deduce 
the rule that 

Increasing the diameter decreases the friction or resistance 
offered to a given flow, since the water is thus caused to 
move more slowly. 

Another equation enters the matter, however, viz., the 
length of the pipe. Since resistance results largely from 
friction with the walls of the pipe, it follows that the longer 
the pipe the more friction there will be. We have already 
seen that with a given volume of water flowing, as the diam- 
eter of the pipe is decreased the friction or resistance is 
increased, and conversely, as the diameter of the pipe is in- 
creased the friction or resistance ,is decreased. 

It is very evident also that w r ith a given flow : 

As the length of the pipe is increased the friction or re- 
sistance is increased. Conversely, as the length of the pipe 
is decreased the resistance becomes less. 

We may therefore increase the resistance by (1) increasing 
the flow of water, (2) decreasing the diameter of the pipe, 
(3) increasing the length of the pipe, (4) increasing the in- 
terior roughness. 

We may decrease the resistance by (1) decreasing the flow, 
(2) increasing the pipe diameter, (3) decreasing the length of 
the pipe, (4) making the interior pipe walls more smooth. 

We believe the foregoing is simple enough to be readily 

What has been said of the action of water flowing through 
pipes is also true with relation to current flowing through or 
over wires. 

If you substitute circuits of wire for the water main, and 
for pipes A, B and C, with volt meters in place of pressure 
gauges, and with lamps, motors or rheostats instead of the 
open pipe ends, you will get precisely the same relative re- 
sult in loss of pressure (voltage) when current flow is sent 
through the circuits. 

In considering electrical action the student should clearly 
fix in mind the proposition that the voltage or pressure of 
the current has absolutely nothing whatever to do with the 
size of wire necessary to convey the current. We may con- 
vey current at ten thousand or at fifty thousand volts on a 
number 40 wire, which is no larger than a very fine silk 
thread, but the amount of current (amperage) we could con- 
vey on such a wire would be very small indeed. 


In passing through wires electric current encounters re- 
sistance, exactly the same as does water in passing through 
a pipe. A wire of given composition and diameter will con- 
vey a certain number of amperes without excessive resist- 
ance (electrical friction) precisely the same as a water pipe 
of given diameter and wall roughness will convey a certain 
given number of gallons without undue friction or resistance, 
and the point where resistance in the wire begins to rise 
above normal marks the "capacity" of the wire, exactly as it 
does in the case of the water pipe. Beyond that point the 
friction, or resistance becomes excessive, manifesting itself 
in loss of pressure or voltage, which same is dissipated in the 
form of heat. This loss in pressure has been consumed in 
forcing the current against the resistance of the wire, pre- 
cisely as was the case in the water pipe. It therefore follows 

Loading wires beyond their normal capacity is expensive, 
and should be avoided for that reason if for no other, since 
the waste is registered on the meter and you will have to pay 
for it exactly the same as you pay for current used in the 
lamps or motors. 

The loss in electric energy, however, is not the end of the 
matter, because if you attempt to force a wire in the excess 
of its rated capacity as shown by the underwriter's table 
(see page 70), heat in excess of normal temperature will be 
developed, and if the matter be carried too far (which can 
only be done by overfusing) the wires may get red, or even 
white hot, finally melting and stopping all current flow, and 
perhaps setting fire to the building in the process. Overload- 
ing wires is, therefore, not only expensive, but it is also 

Precisely as was the case with the water pipe, with a given 
current flow the resistance of a wire is decreased as the di- 
ameter of the wire is increased, its length made less, or its 
composition changed to one of greater conductivity and is in- 
creased as the diameter of the wire is made smaller or its 
length increased, or its composition changed to one of lower 

Resistance increases With increased length of wire; or 

As diameter is decreased; or 

As the temperature is increased 

above normal ; or 
As the composition of the wire is 


changed to an alloy having lower 

Resistance decreases. .. .As length of wire is decreased; or 
As the diameter is increased. 
As the temperature is reduced, if it 

be above normal. 
As the composition of the wire is 

changed to an alloy having higher 


NOTE.— The difference in conductivity of different metals makes the 
analogy of water and current action more complete, since it corre- 
sponds to roughness or smoothness of walls of the water pipe. 

Different metals offer varying resistance to electric current 
as follows, taking the resistance of pure silver and pure cop- 
per as 1. 

Copper 1 *18% Nickel Silver 19 

Silver 1 Manganin 24 

Aluminum 1.5 *30% Nickel Silver 28 

Platinum 6 *Advance Wire 28 

Norway Steel 7 *Climax Wire 50 

Soft Steel 8 *Nichrome 66 

*Ferro Nickel 17 

NOTE. — The Driver- Harris Company, manufacturers of resistance 
wires, are authority for these figures. We know of no more reliable 
source for information of this kind. Star (*) indicates Driver-Harris 

In the foregoing table the figures refer to the amount of 
resistance each metal has, as compared to that of pure, an- 
nealed copper. For instance, platinum has 6 and climax wire 
50 times the resistance of pure, annealed copper. 

We have selected for a part of this table metals and com- 
positions in very general use for resistance purposes. It will, 
of course, be understood that the figures given in the tables 
are based on metals and alloys of a certain standard purity, 
but inasmuch as the degree of purity will, in the very nature 
of things vary, the relative resistance will vary accordingly, 
though the variation should not be very great. 

all metals used for electrical work resistance increases as 
temperature increases. This holds true so far as concerns any 
and everything used for the purpose of conducting the electric 
current, except carbon. In the case of carbon the rule is 
reversed and resistance decreases with increase in tempera- 
ture. This is true to such an extent that the carbon filament 


of an incandescent lamp offers about twice the resistance 
when cold that it does when the lamp is burning at normal 

It might also be remarked that as a general proposition the 
resistance of liquids and of insulating materials becomes less 
as their temperature is increased. 

metallic conductors not being constant at all temperatures, 
but increasing with rise of temperature, and vice versa, it 
becomes necessary that the student understand the law gov- 
erning this matter. 

The increase or decrease of resistance of metals to electric 
current is directly proportional to increase or decrease of 

NORMAL TEMPERATURE.— In figuring such matters all 
calculations are based on normal temperature, which is 75 
degrees Fahrenheit or 24 degrees Centigrade. 

The factor which enables us to calculate the resistance of 
metals with relation to temperature is termed the ''tempera- 
ture co-efficient." In all properly constructed tables of re- 
sistance wire the resistance per mil foot of the material is 
given at normal temperature, and the resistance at this stan- 
dard temperature forms the basis of calculation of increased 
or decreased resistance by reason of temperature change. 
The figure given for temperature co-efficient is the fraction 
of an ohm change in resistance for each degree of change 
change in temperature. This co-efficient must be multiplied 
by the number of degrees of temperature change from the 
normal and the result added to or subtracted from- the resist- 
ance at normal temperature, according to whether the ma- 
terial increases in resistance with heat, as in the case of metal, 
or decreases with heat as in the case of carbon, liquids or 
insulating material. 

For example, let us assume the temperature co-efficient of 
a material to be .001, and that its resistance at normal (75 
degrees F.) is 10 ohms per mil foot. What will be its resist- 
ance per mil foot at 175 degrees? Subtracting 75 (normal 
temperature in degrees) from 175 (working temperature) we 
find the difference to be 100 degrees, and since resistance in- 
creases .001 of an ohm for each degree of increased tempera- 
ture, for 100 degrees increase of temperature the increase of 
resistance would be .001 x 100 = .1 of an ohm. We now mul- 
tiply the resistance at normal (10 ohms) by the fractional 
increase .1, which gives us the actual total increase of 10 x .1 


= 1 ohm, so that the resistance of 175 degrees F. will be 10 
ohms -f 1 ohm == 11 ohms. 

It is not our intent or purpose to do anything more than 
show the projectionist how the temperature co-efficient oper- 
ates. It is not likely he will have occasion to actually use 
it in practice, but it is nevertheless necessary that he under- 
stand the principles upon which such things operate. Those 
who desire further information along these lines can secure 
from their public library books treating on resistance 

PROPERTIES OF CONDUCTORS.— Ordinarily electric 
conductors are selected with one of two ends in view. In one 
case low resistance, tensile strength, ductility and cost are 
the ruling factors. In the other case a comparatively high 
and non-fluctuating resistance is the important item. 

In the first instance conductors for current distribution is 
what is wanted, and it is by reason of the fact that copper 
more nearly combines the four above named important factors 
than does any other metal that it has been selected as the 
standard electrical conductor. 

In the second instance, a material to offer resistance is the 
thing desired, rheostatic resistance forming an integral part 
of electric circuits under some conditions. 

als now most generally used for rheostatic resistance in pro- 
jection arc circuits are either cast iron, made up in grid form, 
or some one of the nickel steel resistance wires. It is very 
difficult, not to say impossible, to secure reliable data con- 
cerning the properties of cast iron, but it nevertheless forms 
an excellent and cheap resistance medium where the tem- 
perature co-efficient may be subject to some variation, and 
where a large difference between resistance at normal and 
resistance at high temperature is not of great importance. 
The resistance per mil foot is 64,3 ohms at normal. Climax 
resistance wire made by the Driver Harris Company, Harri- 
son, New Jersey, has a resistance per mil foot of 500 ohms 
at normal. Its temperature co-efficient is .000543. Climax 
wire is a nickel steel alloy and a most excellent material for 
rheostat coils. 

Eighteen per cent nickel silver is a composition containing 
18 per cent of nickel. Its resistance varies somewhat with 
different lets. Its mil-foot resistance runs from 170 to 180 
at normal. Its temperature co-efficient is .00015 per degree 
F. Ferro nickel has a mil-foot resistance of 170 ohms at 


normal; temperature co-efficient .00115 per degree F. 
Nichrome, also a Driver Harris product, is a practically non- 
corrosive alloy having a melting temperature of about two 
thousand six hundred degrees F. Nichrome is designed for 
use where high temperatures are the rule, as in heating coils, 
etc. Its mil-foot resistance is 660 ohms, and its temperature 
co-efficient point .000095 per degree F. 

Advance wire, a Driver Harris product, is a copper-nickel 
alloy containing no zinc. It is claimed to be constant in its 
resistance under all conditions of service, hence it has no tem- 
perature co-efficient. Its mil-foot resistance is 294 ohms. It 
is particularly recommended for electrical installations where 
resistance is subjected to repeated heating and cooling. 

We are indebted for the figures contained in the foregoing 
to the Driver Harris Company, than whom we know of no 
better authority from which to secure reliable data concerning 
resistance materials. Our intention in dealing with this mat- 
ter has been merely to give our readers some understanding 
of how temperature affects resistance, and how the resistance 
of a material may be calculated with accuracy for any tem- 
perature, providing its temperature co-efficient and its resist- 
ance at normal be known; also to advise projectionists and 
theatre managers of at least one source from whence reliable 
resistance materials may be had in case of emergncy, and the 
names and peculiar qualities of the various metals obtainable 
from this source. 

LOSS THROUGH RESISTANCE.— It is essential to intelli- 
gent, efficient work that the projectionist be able to figure 
the resistance of copper circuits. One of the very first duties 
of the up-to-date, progressive projectionist upon assuming 
charge of a projection room would be to determine whether 
or not the projection room circuits, including its feed wires 
are of sufficient size to operate efficiently and economically. 

As has already been pointed out, the overcoming of resist- 
ance consumes voltage, and since all wires offer resistance to 
current flow, voltage will be consumed in (a) proportion to 
the size of the wire, (b) the length of the wire, (c) the tem- 
perature of the wire, (d) the composition of the wire, all 
these various factors interlocking with one another. 

Up to a certain point the resistance of a wire remains con- 
stant, without change. By this we mean that the resistance 
offered to one ampere or to ten amperes will be identical, but 
when the load becomes such that the temperature of the wire 
rises above normal then its resistance also begins to rise, with 
consequent loss in voltage, or power, which loss will be reg- 


istered' on the watt-meter. The voltage consumed through 
excessive resistance caused by overloading the wire repre- 
sents waste. 

HOW MUCH RESISTANCE.— Broadly speaking, the 
amount of resistance allowable in an electric circuit is : 

For the transmisssion of any given amperage the most 
economical condition is one where the line resistance is such 
that the value of the energy wasted in heat in overcoming 
the resistance of the line will be equal to the interest per 
annum on the original cost of the wires of the circuit plus 
the cost of installation. 

This may be adopted as a safe guide. In practice it means 
that if, for instance, the projection room feed wires are offer- 
ing sufficient resistance to cause voltage drop representing 
enough waste energy to more than pay interest on the cost 
of new conductors of larger size, then it will be true economy 
to install the new conductors. 

Under "Figuring Voltage Drop" our readers will be in- 
structed how to determine the voltage drop and loss in any 
given circuit, so that they may apply the foregoing in prac- 
tice : 

WIRE CAPACITIES.— The National Board of Fire Un- 
derwriters, whose ruling must be followed in matters of this 
kind, else insurance cannot be had on the building, has adopt- 
ed the amperage rating recommended by the American Insti- 
tute of Electrical Engineers. This rating is given in wire 
capacity table No. 1, which determines the number of am- 
peres which any ordinary commercial copper electrical con- 
ductor may be allowed to carry. In the wire capacity table 
U B & S" means "Browne & Sharpe" wire gauge, which is the 
standard in this country. It is also known as the "American 
Standard" wire gauge. For reasons why rubber covered 
insulation wires have a lower amperage rating than other 
insulations see page 83. 

Table No. 2 may be used by the projectionist for figuring 
the actual resistance, in ohms, of his various copper circuits. 
For instance, if the projection room feed circuit has a total 
length of 75 feet, and is of No. 5 copper wire, referring to 
table 2 we ascertain the fact that No. 5 copper has .3174 of an 
ohm resistance per one thousand feet, or .0003174 of an ohm 
per foot, and since the circuit is 75 feet long, hence has l c 
feet of copper, the total resistance would be found bv mul- 
tiplying .0003174 by 150. (Continued under tab 1 - 1 • N 



[Note : Tables 1 and 1A are taken direct from the "National 
Electrical Code."] 

Table No. 1. Allowable Carrying Capacities of Wires. 

u w 

Table A 

Table B 

Table C 

B. & S, 


Area in 































































































































































































1 Mil -one one-thousandth (0.001) of an inch. 

Varnished cloth insulated wires smaller than No. 6 may be 
used only by special permission. 

Note: For insulated aluminum allow 84 per cent, of Table 1 
capacity rating. The Board of Fire Underwriters does not 
recognize anything- of less than No. 18 wire and nothing less 
than No. 14 may be used for interior circuit wires. 






Allowable Carrying Capacities 
in Amperes 




< ^ o 

CD <v o 

O § 





Area in 
Cir. Mils 

+-> . a 

3 rt O 

o 5 o 

Table ] 




W 2 M 

7/ 25 







7/ 32 







7( 40 







7 51 
7 64 













7 81 







7 91 














19 61 







19. 72 







19 81 







19 91 














19 114 







37 91 







37 97 




















































































*These individual strands are odd sizes not listed in the American or B. & S. 
Wire Tables. 

Having this data in hand we have only to divide the current 
in amperes by the resistance of the circuit in ohms to get the 
voltage drop. 

Tables 1 and 1A are both correct for any number of 
amperes up to the capacity of the wire, or, in other words, 
until the load becomes great enough to cause a distinct rise 
in temperature. For instance : If you propose to carry only 5 
amperes on a No. 5 wire you would have exactly the same 
total resistance you would have if you carried 50. 

Theoretically this is not strictly true, since there is a rise 
in temperature with any increase in current, but it is true 
in practice, nevertheless, by reason of the fact that with any 
load less than the wire's capacity the temperature rise is too 
slight to have appreciable effect. 



For the convenience of our readers we append hereto table 
No. 2, which gives the resistance of copper wire at normal 


Resistance at 75° 

F., Internationa 



S 2 . 








Ohms per Lb. 

1000 Feet 


























* 0.0004960 








































































































60 95 








26 00 





















































































When figuring copper wire resistance, still another equation 
enters, however, and a very important one, too, viz., drop in 

ing of the resistance of a wire of any size or length is a sim- 
ple matter, providing the standard of resistance for that par- 
ticular material be known. 

The accepted standard of resistance is the resistance of a 
wire one circular mil in cross-section (one one-thousandth of 
an inch in diameter) and one foot in length, made of the 
same material as the wire it is purposed to measure. This is 
what is known as the "Mil-foot standard of resistance." The 
resistance of such a wire, when made of ordinary commercial 
copper, is given by standard text books as 10.5 ohms. That 
is to say, a wire one foot in length and one one-thousandth 
of an inch in diameter (one mil area of cross-section), made 
of ordinary commercial copper, at normal temperature (75° F. 
or 24° C), will have a resistance of 10.5 ohms. 


now let us proceed to apply the foot-mil standard in measur- 
ing wires. Suppose you have a wire 400 feet in length and 
1 circular mil cross section (1/1000 of an inch in diameter), 
made of ordinary commercial copper. It is evident that if 
one foot of such a wire has a resistance of 10.5 ohms, 400 feet 
would have a resistance four hundred times as great, or 
10.5x400=4200 ohms. The resistance of a wire of given 
length, however, decreases as its diameter or area of cross- 
section is increased. If our 400-foot wire has a diameter of 
250 mils, it will have a cross-section equal to 250x250=62,500 
CM., and it follows that its resistance would be equal to the 
resistance of 400 feet of one-mil wire (4,200 ohms) divided by 
the CM., cross-section of the larger wire (62,500), since it 
would be, in effect, equal to 62,500 wires, each one circular 
mil in cross-section, or one mil in diameter. From this we 
get the rule : 

To find the resistance of a copper wire, multiply its length 
in feet by 10.5 and divide that product by its area in circular 

In measuring circuits, however, it is customary to take the 
one-way length and double the mil-foot standard, thus: mul- 
tiply the one-way length of the circuit by 21 (10.5x2=21) and 
divide that product by the area of the wire in the circuit; ex- 
pressed in circular mils. 


For example: What is the resistance of a two-wire projec- 
tion room feed circuit 75 feet in length — size of the wire No. 
5? If we were measuring only one 75-foot-long wire we 
would apply the above rule, using 10.5 as the standard of re- 
sistance, but as a matter of fact a circuit 75 feet long has 150 
feet of wire, and for convenience's sake we double the mil-foot 
standard, instead of doubling the wire length. 

In Table 1, page 70, we find that No. 5 wire has a cross- 
section of 33,100 CM. We then have the problem : 

Length of circuit x 21 75 x 21 

= = 04.75+ of an ohm, which is 

CM. area of wire 33,100 

the resistance of the circuit. This rule is, of course, based on 
the proposition that the wire will not exceed 75 degrees F., 
or 24 degrees C However, the rise and fall in temperature 
caused by ordinary climatic conditions is not sufficient to 
materially affect the result. In fact, resistance does not begin 
to rise appreciably until the temperature has increased suf- 
ficiently to be sensible to the feeling; beyond that point it 
increases very rapidly with the temperature. 

FIGURING VOLTAGE DROP.— We think it advisable to 
provide the accepted formulas for figuring voltage drop, even 
though the projectionist and the theatre manager may only 
have use for them on rare occasions. 

The question of voltage drop is given altogether too little 
consideration in theatres. In the following formulas 

L stands for one-way length of circuit, 

A stands for cross-section in circular mils. 

e stands for voltage drop, in volts. 

E stands for voltage of circuit. 

I stands for current in amperes. 

R stands for resistance in ohms. 

P stands for voltage drop, expressed in percentage. 

Where the length of the circuit, the area of cross-section of 
the wire, together with its mil-foot standard of resistance, is 
known, the ohmic resistance may be calculated according to : 

21 xL 

Formula No. 1 : R = 


in which 21 is a constant equal to twice the resistance of the 
mil-foot standard for copper wire. Twenty-one and the one- 
way length of the circuit are used, instead of 10.5 and the 


total length of the two wires, merely for the sake of con- 

Example : What is the resistance of a circuit of No. 6 cop- 
per wire having a one-way length of 200 feet? Using formula 
No. 1, substituting figures we have 
21 x 200 4200 

= = .16 of an ohm. 

26,250 26,250 

Formula No. 2 — e = I x R, meaning that voltage drop equals 
amperes multiplied by resistance of the circuit. Example : 
What is the voltage drop of a circuit of copper having .16 
of an ohm resistance and carrying 50 amperes? Substituting 
figures we have: e= 50x.l6, or an 8 volt drop. 
21 x 1 x L 

Formula No. 3 : e = , in volts. Example : What 

is the voltage drop of a copper circuit of No. 6 wire having 
a 200 foot one-way length, carrying 50 amperes? Substitut- 

21 x 50 x 200 

ing figures we have : e = = 8 volts drop. 

21 x 1 x L 

Formula No. 4: A = , in circular mils. Ex- 

ample : What size of wire (area of cross-section) is necessary 
to give an 8 volt drop in a copper circuit having 200 feet one- 
way length, carrying 50 amperes? Substituting figures we 
21 x 50 x 200 

have : A = = 26,250 circular mils. 

Formula No. 5. When the voltage drop is expressed in 
percentage the following formula may be used to determine 
the area of cross-section of wire necessary to give the de- 
2100 x 1 x L 

sired drop. = area of cross-section in circular 

E x P 
mils. Example : Suppose you want a circuit having a one- 
way length of 60 feet to carry 100 amperes with a 3 per cent, 
voltage drop; voltage of circuit 110. Substituting figures we 
2100 x 100 x 60 

have : , which equals 38,181 circular mils, and 

110 x 3 
since a No. 5 wire would be too small (see table No. 1) we 


would have to use No. 4, which would be a little too large 
and would not give quite the 3 per cent. drop. 

Formula No. 6. — If the power is given in watts, the re- 
quired area of cross-section of wire to give a desired voltage 

2100 x W x L 

drop may be figured thus : equals area of 

P x E 
cross-section in circular mils, W standing for watts, of 

And now let us apply the foregoing in practice. A two- 
wire projection room feeder supplies 50 amperes at a distance 
of 200 feet from the house switchboard; we will assume that 
a 5 per cent, drop in voltage is allowed, the supply voltage 
being 110. What size wire should be used? Referring to the 
formula we select No. 5, and, substituting figures, the nec- 
essary size of wire is found as follows : 
2100 x 50 x 200 

A = = 38,181 circular mils 

110 X 5 
Turning to our capacity table we find that a No. 5 wire has 
an area of 33,088 CM. and a No. 4 has 41,740, so that while 
a No. 4 would be largely in excess of the requirements, a 
No. 5 would be too small. 

If this energy were used for ten hours a day for 300 days 
and the cost of the energy were 8 cents per k.w. hours, the 
total yearly cost would be : 

50 x 110 x 300 x .08 

= $1,320, 

5 per cent, of which is $66, which latter amount would 
express a yearly loss due to the 5 per cent, drop when 
using 50 amperes at 110 volts, and at a cost of eight cents 
per K.W. Once the nature of the problem is understood it 
is an easy matter to determine the best course of procedure. 
Sixty-six dollars represent 8 per cent, on approximately 
$800, so that if larger conductors can be installed for that 
sum, or less, and the loss thus reduced, it certainly will pay 
to do it, since paying out sixty-six dollars a year for wasted 
electric energy is precisely the same as paying that sum 
out in the form of interest. 

Of course, we have assumed an arbitrary current cost of 
eight cents per K.W. If current is had for less, the figures 
will be changed. We have, we hope, made the nature of the 
problem clear, and having done that must leave it to the 


individual projectionist and exhibitor to figure out his own 

The data supplied is important, because by its intelligent 
application it will very often be found that much money is 
wasted in the excessive resistance of circuits, which could 
be avoided by installing wires of larger size. It is therefore, 
we repeat, essential that the projectionist and manager have 
a good working knowledge of matters of this sort. 

THE MEASUREMENT OF WIRES.— The area of cross- 
section of electrical conductors of various kinds is measured 
either in square or circular mils, the latter being used for 
round wires and the former for square or rectangular con- 

A circle measuring 1/1000 of an inch in diameter is called 
a "circular mil," the same being commonly abbreviated "CM." 

A round wire 1/1000 of an inch in diameter is said to have 
an area of cross-section of one circular mil, "cross-section" 
being the surface of the end of a wire. 

The area of round wires is directly proportioned to the 
square of their diameters, the calculation being made in 
circular mils. 

"Squaring the diameter" means multiplying the diameter 
by itself, thus : if a wire be 10 mils in diameter, then 10 x 10 
= 100 is the square of its diameter, hence the area of its 
cross-section in CM. 

Let us consider a wire having a diameter of Y^ of an inch. 
Since one inch is equal to one thousand thousandths (1000/ 
1000), the diameter of the wire expressed in thousandths of 
an inch, or mils, would be equal to 1000 -4- 4 = 250. A wire 
% of an inch in diameter is, expressed after the electrical 
fashion, 250 mils in diameter, and since the area of cross-sec- 
tion of a wire in circular mils is equal to the square of its 
diameter in mils, it follows that our quarter-inch-diameter 
wire would have an area of 250 x 250 = 62,500 circular mils. 

The circular mill area of any round wire may be found 
by measuring its diameter in thousandths of an inch, using 
a micrometer caliper or wire gauge for the purpose and 
multiplying the measurement thus obtained by itself. The 
result will be the CM. area of the wire. 

The capacity of any round wire may be found by measur- 
ing the wire diameter as above set forth, multiplying the 
measurement by itself and comparing the result with "Area 
in CM." column in wire capacity table, page 70. 



B. & S. WIRE GAUGE.— The accepted standard for wire 
measurement in this country and Canada is the American 
gauge, commonly known as the "Brown & Sharpe" gauge, 
which in practice is dubbed the "B. & S." gauge. 

This gauge is illustrated in Fig. 7. In using the tool it 
is the slot, and not the round hole which determines the 
size of the wire. In using the gauge select the slot in 
which the wire fits snugly, without binding. A wire gauge 
of this type should have the width of the slot, or in other 

Figure 7. 

words the mil diameter of the wire which fits the slot, 
stamped opposite each slot on one side of the gauge, and 
the number of the wire which fits the slot on the other side. 
For example, opposite the slot in which a No. 16 wire fits 
will be found the No. 51, meaning 51/1000 of an inch, or 51 
mils diameter, the terms thousandths of an inch and mils being 
interchangeable. Note : This is not exactly accurate. The 
precise measurement is 50.0820, but there would not be room 
on the gauge to stamp such long numbers legibly. 

MICROMETER CALIPER.— Wires may also be measured 
with absolute accuracy by means of a micrometer caliper 
such as is illustrated in Fig. 8. 

This tool is, however, expensive; moreover the man un- 



accustomed to handling such tools would have difficulty in 
using it. Micrometer calipers made for the use of electri- 
cians have the wire size and their equivalent in mils stamped 
thereon. Thus: Looking at Fig. 8, if a wire measures 31.9 
thousandths of an inch in diameter, we see that it is a number 
20 wire. If it measures 162 thousandths of an inch (mils) 
we see that it is a number 6 wire, etc. 

For measuring very small wires, such as the strands of an 
asbestos covered wire (usually a No. 30 or No. 31) the slot 
wire gauge is reliable only in the hands of an expert. If the 
projectionist desires to measure his asbestos covered wire 


strands and he has no micrometer caliper, it would be better 
to have a machinist measure a few of the strands with a 
micrometer. Every projectionist should own a wire gauge 
or micrometer caliper. If the former, it should be a good 
gauge, not one of the unreliable catch-penny affairs which 
are worse than nothing at all. The Brown and Sharpe Stand- 
ard Wire Gauge for copper wires, illustrated in Fig. 7, is 
thoroughly reliable. It will serve all the purposes of the 
projectionist, except for the accurate measurement of very 
small wires. 




INSULATION is for the purpose of confining the electric 
impulse, cu/rent or E. M. F. to the wire. Its purpose is 
to prevent electrical contact between wires of opposite 
polarity attached to the same generator. Put in other words, 
the purpose of insulation is to keep the wires from coming into 
electrical contact with each other, or with any object which 
might furnish an electrical path of conductivity to a wire of 
opposite polarity attached to the same dynamo or battery. 
Such a path of conductivity may be supplied by direct contact 
between two wires, by means of both wires coming into con- 
tact with a third wire or other object which will conduct elec- 
tricity, or y both wires making contact with the ground. 
In short, insulation is to protect the potential of or on a 
wire from escaping to a wire of opposite polarity. 

As has already been shown, various metals offer resistance 
in varying amount to the passage of electric current. It 
is also true that various materials other than metals offer 
a varying resistance to the passage of electric current, and 
while there is no material known which is an absolute non- 
conductor — through which the electric current cannot be 
forced if the voltage is raised sufficiently high — still there 
are materials which are considered as being and which 
are treated as non-conductors, because current cannot be 
forced through them by any ordinary commercial voltage. 
These substances are called "insulating materials" and at their 
head stands, in the order named, glass, porcelain and rubber. 

Various natural substances, such as marble and slate, form 
excellent insulating materials for ordinary voltage. Asbestos, 
when dry, is also a very good insulator. Then, too, there are 
various insulating compounds, the composition of some of 
which are trade secrets. In practice these compounds are 
used to saturate some sort of braid or other material which 
after being so saturated is used for weatherproof insula- 
tions on wires to be used out of doors or to re-enforce the 
insulation of rubber covered wires. 

R. C. AND WHAT IT IS.— Rubber covered wire consists 
of copper wire which has been coated with tin, upon which 


is laid a covering of pure rubber or rubber compound of 
homeogeneous character, over which is placed one or more 
outer coverings of braided cotton which have been soaked 
in a preservative fireproof insulating compound. Where 
copper wire is covered with rubber, or any of the rubber 
compounds, the tinning of the wire is very necessary, since 
the sulphur, universally present in rubber insulation is lively 
to combine with the copper, in which case the wire would in 
a very short time become corroded, and thus either very 
greatly weakened or perhaps entirely destroyed. The tinning 
of the wire prevents this, since because tin will not combine 
with sulphur, the rubber insulation has no effect upon it. 

It is not the purpose of this work to enter into an exhaus- 
tive treatise on insulating materials, which subject would 
in itself fill a large volume. Our intent is merely to give the 
projectionist a general understanding of the proposition as 
a whole. Those who wish to study the matter of insulation 
exhaustively should visit their public library and consult 
standard electrical works which deal with insulation. 

The current must be confined to the wires and made to 
pass from positive to negative through the paths provided, 
and through them only, the said paths being motors, in- 
candescent lamps, arc lamps, et cetera. The ability of insula- 
tion to resist electrical action must increase with increased 
voltage, and its kind or type must vary with the service. 
The insulation known as "weatherproof" may be used 
wherever wires are stretched in an open area, and for out- 
of-door circuits, but for interior work only "rubber covered" 
wires may be used. 

INSULATION RESISTANCE.— Where a test of the wiring 
of a building is required by the Inspection Department the 
wiring must comply with the following requirements : 

The complete installation must have a resistance between 
conductors and between all conductors and the ground (not 
including attachments, sockets, receptacles, etc.) not less than 
that given in the following table : 

Up to 5 amperes 4,000,000 ohms. 

10 " 2,000,000 

25 " 800,000 " 

50 " 400,000 " 

100 " 200,000 " 

200 " 100,000 " 

400 " 50,000 " 

800 " 25,000 

1,600 " 12,500 " 

The tes 

must be made with all cut-outs and safety devices 


in place. If the lamp sockets, receptacles, electroliers, etc., are 
also connected only one-half of the resistance specified in the 
the table will be required. 

are two types of weatherproof wire, viz., weatherproof and 
slow burning weatherproof. The insulation of the slow burn- 
ing weatherproof consists of two coatings, one of which is 
fireproof and the other not. The fireproof coating is on the 
outside and comprises about 6/10ths of the total thickness 
of the insulation. The complete covering for size of wire 
from No. 14 to 0000 varies from 3/64ths to 5/64ths of an inch. 

Fireproof insulation is not as susceptible to the action of 
heat as is ordinary weatherproof, which latter softens quickly 
under its influence. Fireproof insulation is not, however, 
suitable for outside work, being intended for interior work 
in warm, dry places, such as shops and factories. When so 
used, underneath it, next to the wire there must be a coating 
of rubber. 

Slow burning insulation, which is still more fireproof than 
the slow burning weatherproof, is intended to be used in 
very hot places, where ordinary insulation would soon perish. 
Weatherproof insulation should consist of at least three 
layers of braid, each thoroughly saturated with a dense, 
moisture-proof compound, applied in such manner as to 
drive out any atmospheric moisture contained in the material, 
thus securing a covering which will not only have high 
insulating power, but which also will to a great extent be 
waterproof. The outer covering of this insulation is pressed 
down to a hard, dense surface. 

Wire thus insulated is intended for use out-of-doors where 
there will be moisture and where fireproof qualities are not 
necessary. In general, weatherproof wires may be used 
only where the supports on which the wire is mounted are of 
insulating material and are depended upon for insulation, 
the covering being regarded merely as a precaution against 
accidental contact with other wires, or other objects. 

In addition to these there is a varnished cloth insulation 
which may only be used in places free from moisture. 

From the foregoing we may understand that the principal 
difference between rubber cover and other insulation lies 
in the fact that rubber cover insulation may be depended 
upon to do the actual insulating, whereas the other must 
depend, at least to a considerable extent, on the wire support 
itself for insulation. Rubber covered wire may be used any- 


where weatherproof would be allowable, but not in places 
where slow burning weatherproof or fireproof insulation 
would be required. 

Rubber covered wire of size No. 8 or less need have but one 
layer of braid and one braid wire No. 8 or less in size may 
be used in conduit. R. C. wire of greater size than No. 8 must 
have either two layers of braided material or a layer of 
tape with the rubber and a layer of braided material out- 

The reason rubber covered wire is rated at lower capacity 
than weatherproof is that rubber is easily and quickly injured 
by even a moderate amount of heat. Remembering that the 
passage of the current through wire generates heat in over- 
coming resistance, it will be readily seen that where an in- 
sulation which is easily injured by heat is used a wide margin 
of safety must be maintained. 

WARNING. — R. C. is the only insulation permissible for use 
in conduit. 




Wire Systems 

TWO- WIRE SYSTEM.— The projectionist of today is not 
likely to come in contact with any except the two arid 
three wire systems. It is true that the "Series Arc Sys- 
tem" is still in use for street lighting, but it may be disposed 
of, so far as the purposes of this work be concerned, with the 
remark that it is not practical to connect a projection arc 
lamp to it. Should the projectionist encounter a series arc 
lighting system the only thing he can do is to let it severely 
alone. Should he attempt to connect a projection lamp to it, 
he will most likely put the whole system out of business, and 
may get himself severely hurt, or even killed. 
The "Multiple Arc" or "Two Wire System" is illustrated in 

Figure 9. 

Fig. 9, in which the heavy lines represent street mains coming 
directly from the power house, circuits D — D branch mains 
feeding a district or a street, and the thin lines E — E — E house, 
store, factory, theatre circuits, etc. We see a projection lamp 
attached to one of these latter circuits, all switches, fuses, 
etc., being omitted. 

Assuming the system to be charged with ordinary commer- 
cial voltage, we may attach a projection lamp to the wires at 


any desired point, provided (a) the wires, switches, fuses, etc., 
be large enough to carry the current necessary for the arc, 
plus whatever else they will have to carry, without over- 
load; (b) provided sufficient resistance be connected in series 
with the lamp to supply required number of amperes. 

In the foregoing, by "commercial voltage" we mean anything 
up to, say, 250 volts, though it is practical to handle even as 
much as 500 volts by means of rheostatic resistance, and that 
is as high as the voltage will ever reach on any D. C. cir- 
cuit. If the current be A. C, then a projection lamp may be 
attached at any desired point, provided the same precautions 
be taken as before named for D. C. ; but if the voltage be in 
excess of, say, 110, you then should only attach your projec- 
tion lamp to the secondary circuit of a transformer, which will 
automatically reduce the voltage line to a pressure suitable 
for use. This latter will be explained in detail under the 
heading of 4 Transiormers," page 596. 

In this connection, let it be remarked that the traveling pro- 
jectionist will do well to procure a copy of McGraw's Elec- 
trical Directory, which is for sale by the McGraw Publishing 
Co., 239 West Thirty-ninth street, New York City. This book 
is issued yearly. It gives the necessary particulars concerning 
every electric generating plant in the country, such as the kind 
of current, voltage of the system, its capacity, etc. 

We shall not remark further upon the two-wire system be- 
cause it is in effect also dealt with in the three-wire system, 
which is the one most commonly encountered and which is 
to a very large extent merely the joining of two two-wire 
systems; therefore, if we went into extended detail on the 
two-wire system, much of what we would have to say on the 
three-wire system would be in the nature of a repetition, and 
there are too many things demanding attention to waste 

THREE-WIRE SYSTEM.— The most widely used system 
for the distribution of light and power is what is known as 
the "three-wire system,'' illustrated in Fig. 10. The basic prin- 
ciple upon which this system operates is the fact that if two 
dynamos of the same characteristics be connected in series 
— the positive pole of one machine electrically connected to 
the negative pole of the other — the voltage between the posi- 
tive and negative terminals of the combination thus effected, 
or in other words, the potential difference between the outer 
poles or terminals of the two machines, will be double that 
of either dynamo measured separately. 


If each dynamo be a 110-volt generator, then the pressure 
between the outer terminals and the wires connected to them 
(wires E and F) will be 220 volts, though if at the same time 
the voltage be taken across the positive and negative terminal 
of either dynamo separately (between wires E and D or D»and 
F), the reading would only be 110. It therefore follows that 
if a wire be attached to the outer terminals of two generators, 
as per wires E — F, Fig. 10, in which circles A and B represent 
110-volt dynamos, and the other two terminals be joined by 
wire C, as shown, the voltage between these two wires will 
be double the voltage of either machine separately. If a 

Figure 10. 

third wire, D, be attached to connecting wire C, the voltage 
reading between, either wires E — D or D — F, would be half 
that between wires E and F. In such a combination the center 
wire D is called the "neutral." The combination is such that 
current from either dynamo may be used without in any way 
affecting the other dynamo. Two outside wires E — F, are 
known respectively as the "true" negative and "true" positive 
of the combination, but 

Neutral wire D is negative to dynamo A and positive to dy- 
namo B, so that as a matter of fact it is both positive and 
negative, and when using current from either dynamo sep- 
arately it will be either a positive, or a negative wire 
according to from which dynamo the current is taken. 

If we connect a lamp or motor to wires E and D, then wire D 
will be just as truly negative as though dynamo B did not 


exist. If we connect to wires D and F, then wire D will be 
just as truly positive as though dynamo A did not exist. 

G — H and I are voltmeters. Voltmeters G and H will each 
register 110 volts and voltmeter I will register 220 volts. 

Put in another way, in the three-wire system we have what 
is in effect two complete two-wire systems joined together in 
such way that they may either be used separately at 110 volts, 
or jointly at 220 volts. The reason for such a combination 
is that it is economical in installation cost and maintenance, 
since the same electrical energy can be transmitted over a 
three-wire system that could be conveyed over two separate 
two-wire systems having wires of equal size; also, there is the 
added advantage of being able to use 110 volt incandescent 
lamps (which are very much better than 220 volt lamps) and 
either 110 or 220 volt motors. 

Assuming the system shown in Fig. 10 to be in operation, 
its electrical action will be as follows : First let us switch 
off lamps J, L and N, leaving only lamps K and M burning. 
Assuming the lamps to consume 55 watts each, the amperage 
passing through any one of them when burning alone on 110 
volts pressure would be 55 -s- 110 = .5 of an ampere; also, 
the amperage passing through two such lamps when burning 
in series at 220 volts would be 110 -*- 220 = .5 of an ampere, 
each lamp consuming 55 watts or 110 watts in all. 

A peculiarity of the three-wire system is that lamps or 
motors connected to opposite sides will always, when possible, 
operate in series, the current always seeking the true negative 
rather than the neutral wire. 

It therefore follows that with only lamps K and M burning, 
they would burn "in series" at 220 volts' pressure, hence J4 
ampere of current would pass out from generator A and 
along wire E to lamp K. The current would pass through 
lamp K to the neutral wire, 110 volts of its E. M. F. being con- 
sumed in forcing the current through the resistance of the 
lamp filament, along the neutral wire to lamp M, through 
lamp M and back to generator B along wire F, the true 
negative of the combination. Under this condition no current 
at all would pass over the neutral wire, except between the 
points at which lamps K and M are connected to it. 
1 Let us now switch on lamps L and N, whereupon instantly 
one ampere of current will follow along wire E up to the 
point where lamp K is connected to it. At this point the 
ampere will divide, one-half going through lamps K and M 
the other half continuing on and passing through lamps L 


and N, so that we now have one ampere flowing on the true 
negative between the generator and the point where lamp M 
is connected to wire F. The combination is now producing 
one ampere of current under 220 volts' pressure and is what 
the electrician would call "perfectly balanced." 

BALANCED SYSTEM.— A three-wire system is said to be 
"balanced" when lamps or motors consuming precisely the 
same amount of current are attached between the neutral 
and either of the outside wires. In other words, when each 
"side" is carrying exactly the same load, under which con- 
dition everything operates in series and no current flows on 
the neutral between the generator and the first current using 
device attached to it. So long as this condition be maintained, 
the power-house neutral fuse could be removed without in 
any way affecting the system. 

From the power-house viewpoint a perfectly balanced three- 
wire system is highly desirable. This ideal condition is, how- 
ever, seldom or never realized in practice. There is prac- 
tically always more load on one side than on the other, and 
amperage equal to the difference between the load on the 
two sides flows back to the generator on the neutral wire. 
If the system is a 110-220 volt one, and 26,400 watts are being 
used on one side and 24,200 on the other, then 26,400 minus 
24,200 equal 2,200 -^ 110 = 20 amperes will flow back to the 
generator on the neutral wire. The practical effect of this 
would be that one generator would produce 20 amperes 
(2,200 watts) more than the other. 

For this reason officials of heavily loaded three-wire systems 
often object to projection arcs being connected to one side of 
the system. Both the dynamos are working close to capacity, 
and if a projection arc, which is in the nature of an inter- 
mittent load of considerable amount, be hitched to one side, 
that amount of load is thrown on one dynamo and the system 
is thus intermittently unbalanced. 

However, if the projectionist is reducing his voltage with a 
rheostat there would be no advantage in connecting the pro- 
jection arc across the outside wires, since although the 
amperage would remain the same, an amount of energy equal 
in watts to an additional arc of equal capacity would be con- 
sumed in the resistance necessary to take care of the extra 
110 volts. Thus, instead of having one dynamo intermittently 
loaded with a projection arc, both generators would carry 
an additional load equal in watts to the arc amperage times 



If, however, an economizer, a mercury arc rectifier or a 
motor generator set be used for voltage reduction, then there 
is large advantage in connecting to the two outer wires, since 
there would then be no unbalancing effect and the total 
energy taken from the lines would be practically the same as 
when connected to one side at 110 volts. It may be accepted 
as fact that, 

If the line voltage be reduced by means of a rheostat, the 
power company can have no reasonable excuse for compelling 
you to attach your projection arc to the outside wires of a 
three-wire system. Such a connection would cost exactly 
twice as much for electrical energy to operate the arc as it 
would cost if you were connected to one side at 110 volts, and 
places double the load on the system. 

But to return to the consideration of Fig. 10, we have seen 
that with lamps K, L, M and N burning the system is balanced 
and no current flows back to the generator over neutral 
wire D. Let us now turn on lamp J in addition to lamps K. 
L, M and N. This will cause V/ 2 amperes to be generated, 
which will flow out on wire E to the point where lamp J is 
connected, whereupon it will divide up as shown, V 2 ampere 
passing through lamp J, y 2 through lamp K and Y 2 through 
lamp L. Between wires D and F y 2 ampere will flow through 
lamp N and y 2 through lamp M, but there is no lamp to 
balance lamp J, therefore the current flowing through it must 
return to the generator over neutral wire C. Hence we now 

+ /. 










Figure 11. 


have the system unbalanced by y 2 ampere, with 1J4, y 2y 1 
ampere flowing respectively in wires E, D and F between the 
lamps and the generators, and generator A producing y 2 
ampere more of current than generator B. 

Insofar as current flow be concerned that is the way the 
three-wire system operates, but there are some points in con- 
nection with it which are very puzzling to the novice and 
more or less so to some more experienced men. 

Fig. 11 is the diagrammatic representation of several house 
circuits fed by a three-wire service circuit, each wire of which 
is fused at 60 amperes. Between the neutral (central) wire and 
the upper wire, circuits A, B and C are connected, each of 
which is connected to apparatus using just 10 amperes. 
Between the neutral and the lower wire circuits D, E and F 
are connected the first two using 10 amperes each. Circuit F 
is idle. 

Question: Would it be possible to attach a 25-ampere pro- 
jection arc to circuit F, when the three main wires are only 
fused to 60 amperes and the circuit already loaded as shown? 

The novice will probably answer, "No, the circuits are 
already using 50 amperes, and the addition of a 25-ampere arc 
would overload the fuses." The novice would, however, be in 
error, because the circuits are not using 50 amperes, but only 
30, 10 of which are handled individually by the generator at- 
tached between the neutral and the upper wire. Circuits A, 
B, D and E will burn in series, as has already been explained, 
so that instead of 40 amperes at 110 volts, the lamps or motors 
on circuits A, B, D and E will work in series on 220, and only 
a total of 20 amperes will flow. 

Circuit C will use 10 amperes at 110 volts, just as though 
wire 3 did not exist, as long as circuit F is idle. This will 
have the effect of causing the upper wire to carry 30, the lower 
20 and the neutral to carry 10 amperes, so that the upper fuse 
will carry 30 amperes, the neutral fuse 10 amperes and the 
lower fuse 20 amperes. Under that condition the system is 
unbalanced 10 amperes, and the generator attached to the 
neutral and upper wire is carrying that much more load than 
is the generator attached to the neutral and lower wire. 

Suppose we now connect a 25-ampere projection arc to 
circuit F. Circuit C now burns in series with circuit F to the 
extent of 10 amperes, but 15 amperes of the 25 must come 
from the generators over neutral wire, so that we now have 
the following condition: The upper fuse carries 30, the center 
fuse 15 and the lower fuse 45 amperes, and the generator 


attached to the neutral, and lower wires is generating 15 
amperes more than its mate. 

We therefore see that instead of being overloaded the fuses 
would actually be too large to properly protect the apparatus, 
were it not for the individual circuit fuses. To be fused 
absolutely right for the protection of the apparatus, we should 
now have a 30-ampere fuse in the upper, a 15-ampere fuse in 
the center and a 45-ampere fuse in the lower contact, though 
this is never done in practice, the fuses shown being intended 
to protect the main wires, the capacity of which would pre- 
sumably be 60 amperes. The apparatus and smaller wires are 
protected by individual circuit fuses not shown in the diagram. 

If your theatre is fed by a three-wire system it is important 
that the two sides be balanced as nearly as possible. If the 
theatre system is unbalanced and the neutral fuse should blow, 
then the effect is that of forcing the lights attached to one 
side above candle power, while those on the other side would 
burn below candle power. 

It is always possible to tell exactly how much, if any, the 
load is unbalanced by connecting an ammeter into the neutral 
house feeder. 

sizes for three-wire circuits, proceed the same as for the 
ordinary two-wire system, considering only the two outside 
wires. Having determined the necessary capacity of the two 
outside wires, make the center wire the same size. 





IT is essential that the projectionist be able to recognize 
the various types of switches met with in theatre work, 
also that he understand certain things with regard to their 
installation and proper care. 

The various types of knife switches ordinarily encountered 
in theatre work are the double pole single throw (D. P. S. T.) 
double pole double throw (D. P. D. T.) three pole single throw 
(T. P. S. T.) and three pole double throw (T. P. D. T.). 

In Fig. 12 we see 
at the top a S. P. 
S. T. knife switch 
mounted on a slate 
insulating base, slate 
being an insulating 
material for ordi- 
nary commercial 
voltages. A is the 
blade, B the handle, 
E E the terminals, 
D the hinge and C 
the contact. Im- 
portant points in the 
care of switches are 
(a) that hinge D be 
kept set up snugly, the proper pressure being that which 
will cause the blades to remain in any position placed. If the 
switch is held upright, and when the blade is pulled out it 
falls down of its own weight making contact with C, then 
hinge D is too loose and should be tightened. The next im- 
portant point is that contact C be so adjusted that there will be 
good, firm electrical contact between the blade and the con- 
tact clips C when the switch is closed. It is amazing how care- 
less some projectionists are about such details. We have 
many times, in high class projection rooms, found switches 
loose and wobbly in their hinges, or making such poor contact 
at C that considerable heat was generated. 

SWITCH INSTALLATION.— In the installation of knife 
switches, such as shown in Figs. 12 and 13, it is important 

Figure 12. 


that the switch be so placed that the tendency will not be for 
the switch to close itself by gravity. This means that the 
upper switch in Fig. 12 should, if mounted on a wall, always 
be placed either sidewise or with its handle up. 

Switches often have fuses mounted on their base. In Fig. 
12 the lower illustration shows an S. P. S. T. knife switch 
with fuse contact at the left, these contacts being designed to 
take what is known as the "knife blade" cartridge fuse. The 
fuses are not in place. 

Figure 13. 


In Fig. 13, A is a D. P. S. T. and B a T. P. S. T. knife 
switch, without fuse contacts; C is a D. P. S. T. knife switch 
with knife blade cartridge fuses, and D a D. P. S. T. knife 
switch with ferrule contact cartridge fuses. For explanation 
of the different kinds of fuse contacts see "Fuses," page 107. 

In Fig. 13, E and F are types of porcelain base D. P. S. T. 
switches, with receptacles for plug fuses. This type of switch 
is called a "panel cutout." It is often used in building up 
panel boards, but may only be used to control individual cir- 
cuits of low amperage. 

ENCLOSED SWITCHES.— An enclosed switch is one 
having an individual protecting cover, usually of sheet metal 
which entirely encloses and protects all "live" parts of the 
switch. All projector switches are and must be enclosed 
switches, no other kind being permitted for this purpose. 
The enclosure of the switch by a metal covering is to protect 
the projectionist from possible shock by accidental contact 
with its live parts, as well as to prevent possible short cir- 
cuits or injury to the switch by contact with various objects. 
It is important that the covering of enclosed switches be so 
made that it cannot come into contact with the live parts of 
the switch. 

In connecting enclosed switches it is better that the blade 
end of the switch be dead when the switch is open. In fact 
that rule applies to all switches, though sometimes circum- 
stances prevent its being adhered to. 

LOCATION OF SWITCHES.— In the location of switches 
local conditions must, of course, largely govern, particularly 
in the smaller theatres, but the house switchboard should be 
so located that the man in charge of it will have an unob- 
structed view of the screen when at the switchboard. Unless 
this be done there is apt to be an imperfect handling of the 
house lighting at the beginning and the end of the show, or at 
other points where change in the auditorium lighting may be 
necessary, no matter what care may be taken to co-ordinate 
the work of the projectionist and the work of the switchboard 

Switches governing emergency lights, which include all 
lights kept burning during the performance, should under no 
circumstances be placed on the main switchboard. You can 
never tell what an excited man will do, and in case of fire 
people inside the auditorium, including the employees, are apt 
to become excited. Some one might pull the emergency light 


switches on the main switchboard, and thus set up a tre- 
mendously dangerous condition. Place the emergency light 
switches in the box office, where nobody can get at them but 
the ticket seller, and make him or her directly responsible for 
their handling. 

In the projection room, local conditions will govern the 
placing of switches, but it should be remembered that nothing 
can possibly be gained by making things inconvenient for the 
projectionist. Wrongly located switches often cause much 
entirely unnecessary labor and annoyance ; also inconveniently 
located switches cause delay, and make the proper handling 
of the program impossible. 

The projection room incandescent lights should, as a whole, 
be governed by one switch, located within convenient reaching 
distance from working position at either projector. This will 
enable the projectionist instantly and fully to illuminate the 
room, or to cut off all lights instantly and conveniently, which 
latter is the best condition for projection. Each lamp socket 
should, however, have an individual snap switch. 

This is of paramount importance, because it is impractical, 
not to say impossible, under conditions usually found in pro- 
jection rooms, to produce the best possible screen results 
with incandescent lights burning, and the projectionist is more 
apt to extinguish his lights if there is a switch handily located 
with which he can put them all out or on with one operation 
than if he has to turn them off by using two, three or more 
switches. This is one of the seemingly unimportant points 
which is of great importance to results on the screen. See 
page 345 for modification. 

USE OF TYPES OF SWITCHES.— Except for very limited 
purposes the use of the single pole knife switch is prohibited 
by underwriters' rules. So far as we are aware, none of the 
purposes for which a single pole switch may be used exists in 
a theatre, except in making certain rheostat connections, as 
will be explained under the heading "Rheostats." 

The D. P. S. T. switch is the type ordinarily used to control 
all incandescent and projection circuits, except those con- 
trolled by triple pole or D. P. D. T. switches. The T. P. S. T. 
is used to control three-wire circuits where they enter a 
theatre, and wherever else the three-wire circuit may extend. 
D. P. D. T. switches are used in certain fuse connections, as 
will be explained under "Fuses." These switches are also used 
for connecting two separate two-wire supply systems, and for 


projection circuit connections under certain conditions. Also 
for polarity changing. 

SWITCH MARKINGS.— It is required by underwriters' 
rules that switches have certain dimensions, according to the 
voltage they are to be used on, and the number of amperes 
they must carry. 

Both the voltage and amperage capacity must be stamped 
on some part of a knife switch. Reject any switches not so 

A switch may be used for a less amperage and less voltage 
than it is rated to carry, but never for a higher voltage or a 
higher amperage, thus : you might use a 500 volt 50 ampere 
switch on a 110 volt circuit and to carry any number of 
amperes up to 50. But you would not be permitted to use a 
switch of less than 50 amperes capacity for 50 amperes, or a 
250 volt switch on a 500 volt circuit. The higher the voltage 
the further apart the blades of the switch must be placed, and 
the longer the switch blades must be. 

Two hundred and fifty volt switches are the type almost 
universally used in theatres. There is no such thing as a 110 
volt switch, the requirements for 110 and 250 being the same. 

RECAPITULATION. — Be certain your switches have suffi- 
cient capacity to carry the amperage. 

Be certain your switches are of proper voltage capacity. 

Be certain your switches are so installed that the handle 
will not move downward in closing the switch. 

Be certain the hinges and contacts of your switches are 
tight and in good condition. If contacts become roughened 
they may be smoothed with 00 sand paper which should be 
wrapped around a thin strip of metal for smoothing the inside 
of contacts C, Fig. 12. 

Be sure the cross bar to which the switch handle is fastened 
is kept firmly attached to the blades. A loose, wobbly switch 
is an abomination; also it is an evidence of a careless, in- 
efficient workman. 

METAL CABINET.— Unless switch cabinets are built into 
the walls, all projection room switches and all other switches 
except those on the stage switchboard should be enclosed in 
a metal cabinet, such as is illustrated on page 104, the same 
to be equipped with a door which automatically closes, either 
by gravity or by a spring. 

MAIN HOUSE SWITCHBOARDS.— Main house switch- 
boards, particularly in medium sized theatres, are frequently 


placed in the projection room, in which case the entire audi- 
torium lighting is under the direct care and supervision of 
the projectionist. He not only handles the switchboard itself, 
but the "dimmers," the latter being what amounts to a series 
of adjustable rheostats by means of which various incan- 
descent circuits in the auditorium may be gradually dimmed 
down and finally extinguished. 

In the "National Electric Code," copy of which may be 
secured by sending five cents in stamps to the National Board 
of Underwriters, Electric Department, 123 William Street, 
New York City, appear the following rules which must be 
strictly observed in the installation of switchboards : 

a. Must be so placed as to reduce to a minimum the danger 
of communicating fire to adjacent combustible material. 

Switchboards must not be built up to the ceiling, a space of 
three feet being left, if possible, between the ceiling and the 
board. The space back of the board must be kept clear of 
rubbish and not used for storage purposes. 

b. Must be made of non-combustible material. 

c. Must be accessible from all sides when the connections 
are on the back, but may be placed against a brick or stone 
wall when the wiring is entirely on the face. 

If the wiring is on the back, there must be a clear space of 
at least eighteen inches between the wall and the apparatus 
on the board, and even if the wiring is entirely on the face, it 
is much better to have the board set out from the wall. 

d. Must be kept free from moisture. 

c. Insulated conductors when closely grouped, as in rear of 
switchboards, must have a substantial flameproof outer 

Flame proofing must be stripped back on all conductors a 
sufficient distance from the terminals to give the necessary 
insulation distances for the voltage of the circuit on which 
the conductor is used. 

As has been already said under "Location of Switches," 
page 94, the location of the main house switchboard will 
depend largely upon local conditions, and may only be properly 
determined by considering the peculiarities of each individual 
case. The best location in one theatre might be the worst 
in another. 

In fixing the location, whether the switchboard be in the 
projection room or elsewhere, the architect or designer should 
be guided largely by the items accessibility and convenience, 
remembering always that if the switchboard be located out- 


side the projection room it is essential it be in such position 
that the man handling it will have a good view of the screen or 
of the stage when at his post of duty. This latter is essential 
to the best manipulation of the lights, particularly if there is 
vaudeville, unless the lights be handled from the stage, as will 
most likely be the case in theatres where there is a stage. 

MATTER OF SAFETY.— If the main house switchboard 
controlling the auditorium lights be located in the projection 
room there should always be an arrangement by means of 
which the auditorium can be lighted from a suitable location 
in the auditorium itself. Also if the main house switchboard 
be located in the auditorium there should be an arrangement 
by means of which the auditorium may be lighted from the 
projection room. 

An emergency may at any time arise in which it is im- 
perative that the auditorium be lighted instantly. This 
emergency may arise in the projection room, as in the case of 
a film fire, and unless the projectionist himself be able to 
switch on the lights, a space of time sufficient to set up a 
dangerous condition might very likely elapse before it could 
be done from the auditorium. It is also possible that an 
emergency would arise in the auditorium itself where safety 
would demand the instant lighting of the auditorium by the 

It is quite possible for the projectionist to signal or tele- 
phone to the main switchboard attendant to switch on the 
lights, or vice versa, but in case of serious emergency the 
delay involved might be sufficient to cause a dangerous con- 
dition ; also the signal apparatus or telephone might not be 
in good working order just at the crucial moment. 

Personally we do not favor the placing of the main switch- 
board in the projection room except under conditions where 
there are always two men present in the room. We have 
long since taken the position, and see no reason to change it, 
that when a photoplay is "on," the projectionist should have 
nothing of any kind whatsoever to do except watch the screen 
and regulate those various things essential to a perfect screen 

If, however, there are two projectionists, as is the case in 
many theatres, or even if there is a projectionist and a helper 
always present in the projection room, then the ideal con- 
dition is to have the auditorium lighting, including the dim- 
mers, handled from the projection room. The projectionist 
may then work entirely from pre-arranged cues in the hand- 


ling of the whole show, including the auditorium lighting, 
and there will be no division of responsibility. A proper co- 
ordination of the picture, the music and the lighting is of 
paramount importance, particularly in houses where the 
music and staging of the picture have been carefully worked 
out. An effect which, if properly worked, would be beautiful, 
may be ruined by just a few seconds delay in the manipula- 
tion of the auditorium lighting. 

We cannot emphasize the importance of this latter too 
strongly. It was Samuel L. Rothapfel who first pointed the 
way to a truly artistic presentation of the photoplay upon the 
screen, and in the scheme of affairs as outlined by him, which 
is now followed, in greater or less degree, in all high class 
photoplay theatres, much depends upon close co-ordination 
of the auditorium lighting with the other various features of 
the program. It is therefore evident that the location of the 
main house switchboard is a matter for careful consideration 
by the management and the architect at the time the theatre 
is built. 

THE "BOARD."— It is essential that both the projectionist 
and the- man-in direct charge of the theatre auditorium have a 
good understanding of the main house switchboard and its 
electrical connections. These switchboards are often imposing 
affairs, but once their connections are traced, they are simple 

The main house switchboard will, or should, carry every 
circuit in the theatre, including the projection arc circuits" and 
stage feeders, excepting the emergency light circuits, which 
latter must be attached to the theatre feed wires ahead of 
everything, including the main house fuses and switch, see 
page 103. 

The main house switchboard will carry the (a) main fuses, 
placed ahead (on the street side) of everything except the 
exit and emergency circuits. These fuses will carry the 
entire house load, except the circuits just named, and except 
the stage, if the stage has a separate set of service wires, 
(b) the main switch, which kills everything but the exit and 
emergency lights, (c) fuses for every individual circuit in the 
house, including the projection room and stage feeders, if the 
latter are attached to the main board, (d) service switches for 
every individual circuit, including projection room feeders 
and stage feeders. 

Of course what the main house switchboard will carry may 
be subject to modification by the peculiarities of the individual 



installation. In small, strictly moving picture houses, in which 
light effects are not attempted, it is much better to have 
auditorium lights that are not used during the show ex- 
tinguished all at one time, rather than by pulling several small 
switches. In large houses, however, where there are many 
incandescent lights and circuits, this is neither a practical nor 
a desirable thing to do. In such houses dimers should always 
be used. 
In figure 14 we have both a digrammatic and photographic 

^'H s s 

Figuie 14. 

representation of a small 3-wire switchboard, commonly 
known as a "panel board." In the diagram, A is the fuse con- 
tacts, B the main switch, C the house circuit fuse contacts 
and D the service switches governing individual circuits. All 
of this is seen photographically represented at the right, 
except that in the photographic representation the main 
switch and fuses are omitted, and there are five circuits on 
each "side," instead of three. Both in the photograph and 
the diagram the screw heads connecting the individual cir- 
cuit feeder bars to the main circuit feeder bars form the key 
to the connection. 

Taking the diagram for example, it will be observed that 
the center or neutral bar has a screw head over the second 
and third individual circuit bars, which means that the 
neutral bar is electrically connected to these two bars, or 
in other words, to the upper bar of the lower circuit and the 
lower bar of the upper circuit. The right hand short bar is 
connected to the lower bar of the lower circuit and the left 
hand feeder bar is connected to the upper bar of the upper 
circuit. It will thus be seen that the lower circuit is con- 


t Ul -43* 





f. .^r^. J 



1 */* 

f ;d 
L fe- 


Figure IS. 


nected to the neutral and the right hand feeder bar, so that 
it is on the right hand "side" of the three wire cuicuit. The 
neutral and the left hand bar is connected to the upper cir- 
cuit, so that circuit is on the left hand "side." We thus have 
one circuit connected to each "side," and if both circuits use 
the same number of amperes the load will be "balanced." , 

This forms the keynote to the connections of your big 
house switchboard. It is a bit puzzling for the novice to 
trace these connections, but look at it for a while and you 
will find that in all individual circuits one side is connected 
to the neutral and the other to one or the other of the main 
switchboard feeder bars, except in the possible case of the 
use of a 220 volt motor circuit, which would connect to the 
two outside bars or wires. It will be understood that where 
there is no screw head there is no connection between the 
feeder bars and circuit bars. Thus : The left hand feeder 
bar, Fig. 14, crosses the lower second and third bars without 
electrical connection and makes electrical connection to the 
top bar at the screw head. Where copper bars of this kind 
are used instead of wires they are commonly called "bus 
bars," though the term correctly applies to the copper bars 
which connect the power house generators to their circuits. 

Fig. 15 is a photograph of a moderately large and some- 
what complicated switchboard. On the right side the in- 
dividual circuits are indicated by X. Study the contacts 
and you will be able to trace out the connections. Taking 
the next-to-the-top right hand circuit for example, we find 
it leaves the main bus bars in the form of a three-wire cir- 
cuit, and that there are three plug fuses which protect the 
three-wire circuit as a whole. Just beyond the fuses is the 
handle of the T. P. S. T. switch, beyond which the upper bar 
connects to the upper wire of the upper two-wire circuit, 
the neutral connects to both the lower wire of the upper 
circuit and the upper wire of the lower circuit, and the lower 
bar connects to the lower wire of the lower circuit. 

We thus have the three-wire circuit split up into two two- 
wire circuits, at the beginning of which are the individual 
circuit fuses which must be present on all individual circuits. 
To the left this circuit starts off and ends as a plain three- 
wire circuit. Above this circuit, at the very top of the bars, 
are two two-wire circuits, the neutral bar (see Screw Head) 
connecting to the lower circuit bar, the left hand bus bar 
to the upper left hand circuit bar and the right hand bus bar 
to the upper right hand circuit bar, and thus by a little 
care in observing the screw heads, which mean electrical 



contacts, we may readily trace out the connections of any 
house switchboard in which the bus bars show on the front 
of the board. If they are at the back of the board it com- 
plicates things a little for the beginner, but the action is 
traced out in the same manner. In Fig. 15 the main fuses and 
the 3-pole switch controlling the whole board are not shown. 


smaller theatres it is an occasional practice to build up a 
switchboard of porcelain base panel cutouts, such as are 
illustrated in Fig. 16, and at E F, Fig. 13. Any number of 
these blocks may be used, and they may be had for either 
two or three-wire circuits, but may only be used for indi- 
vidual incandescent light or motor circuits of low amperage. 

These cutouts 
must always be 
mounted on an 
insulating base, 
and must be pro- 
tected by a sub- 
stantial metal 
cabinet similar to 
that shown in 
Fig. 17. It is per- 
missible to form 
an insulating base 
for these blocks 
by placing either 
sheet asbestos or 
asbestos mill 
board not less 
than Y% inch 
thick at their 
back. At the head of a board of this kind there should be 
a suitable knife switch having capacity equal to that of all 
the circuits of the board. This switch should carry the main 
switchboard fuses. 

If properly put together such a board is just as efficient, 
although it does not look so well as the regular slate base 

exit and emergency circuits must, as has already been set 
forth, be tapped to the main house service wires on the 
street side of the main house switch and fuses. 

These circuits should be controlled by switches located 


Three to Two- Wire Double Branch. 

Figure 16. 



either in the box office or in the manager's office, and by no 
other switches. 

Exit and emergency lights comprise the light and exit 
signs and all lamps in entrance foyer, stairway and other 
parts of the theatre used by the audience either regularly 
or in case of emergency, and are ordinarily left burning 
during the performance. 

For the fusing of these^ circuits see page 120. 

STAGE SWITCHBOARD.— It is not within the province of 
this work to deal with the stage switchboard, except to point 
out a few important elements which are demanded by the 
National Board of Fire Underwriters, and which make for 


5 £I":| j 


B I 

- ,# - •' 




Figure 17. 

The stage switchboard is ordinarily located on the 
proscenium wall. The common practice is to place it to the 
right of the proscenium as one looks towards the audience. 

Stage switchboards should never be installed in any theatre, 
no matter how small, without first ascertaining the Board 
of Fire Underwriters' requirements for such installations. 

It is required that the board be protected by a sub- 
stantially constructed iron railing of certain height, located 
a certain distance from the board, and securely fastened to 
the floor. This guard is to protect the switchboard from 



t/C^T eraser. 

8Q II Q9 

-p Q ii O p 

/ v 




-QG iiUQs 

16 Sr-frQe. ^Y__^ 



8 8 





Figure 18. 


accidental injury by being struck with moving objects, or 
from persons falling against it, as well as to prevent such 
accidental contact causing fire. 

All fuses on a stage switchboard must be of an approved 
cartridge or plug type. It is absolutely forbidden, under 
any circumstances, to use a link or open fuse on any stage 

The stage switchboard should carry main fuses and main 
switch controlling all current in the board. It will, of course, 
carry the various service fuses and switches for each indi- 
vidual circuit. All switches should be plainly marked with 
the name of the circuit they control, thus "white foots," 
u red foots," "blue foots," "first borders white," first borders 
green," etc. 

The utmost care must be exercised that all switch con- 
tacts, etc., be kept in perfect electrical and mechanical con- 
dition, to prevent any possibility of heating which might, 
under some conditions, be extremely dangerous, and the 
whole installation should be carefully examined at regular 
intervals to see that it is in perfect condition. 

Absolutely no one except the man in charge of the stage 
switchboard should under any circumstances be allowed to 
touch it while the performance is going on. The fewer 
people handling it at other times the better. Stage switch- 
boards should always be wired from the back. While this 
is not absolutely demanded, it is safer and in every way 
very much better. 

We do not care to deal further with the stage switch- 
board, since those contemplating the installation of one 
should accept the dictates of no authority except the city 
or state officials and the National Board of Fire Under- 
writers, with whom contact may be always had by ad- 
dressing the National Board of Fire Underwriters, Elec- 
trical Department, 123 William Street, New York City. 

BUILT-UP BOARD.— For those who prefer to build up a 
switchboard by using porcelain base switches, Fig. 18 will 
serve as a guide. For a small board ^ inch asbestos mill 
board makes an acceptable insulating support. Such a board 
may be built up quite inexpensively, and being installed in 
a metal cabinet such as that shown in Fig. 17, which may be 
had of any dealer in electrical supplies, will give very good 
service. The circuits marked X are incandescent circuits 
for the auditorium. 




AN electric conductor of given size will, as has already 
been set forth, carry only a certain given number of 
amperes of current without developing heat above 
normal temperature. See page 66 and Table No. 1 on page 70. 

Ordinarily only the number of amperes consumed by the 
various motors and lamps attached to a circuit will flow over 
the wires of the circuit, and the combined capacity of lamps 
and motors attached to any circuit is never presumed to 
exceed the rated capacity of the wires. Many things, how- 
ever, such as grounds, short circuits or a rise in the voltage 
may occur to cause an abnormal flow of current sufficient 
to overload wires, or if it be a rise in voltage then to over- 
load the apparatus attached to the wires as well. 

The fuse is a sort of electrical safety valve designed to 
act automatically and prevent overload of this kind. 

Figure 19. 

In Fig. 19, we see an elementary set of fuses diagram- 
matically illustrated. The wires of a circuit are cut and 
their ends attached to terminals A-A-A-A, these termi- 
nals being mounted on insulating base B. Between these 
terminals, taking the place of the copper circuit wires, are 
two lengths of "fuse wire/' which is wire composed of an 
alloy of metals, usually lead and tin, having a very low 
melting temperature and a high temperature co-efficient, 
which means that the resistance of fuse wires rises rapidly 
with overload. 


The practical operation is as follows : The current 
capacity of the fuse wire is in no case presumed to exceed 
the rated capacity of the wires of the circuit they protect, 
and only to exceed the combined current consuming 
capacity of the lamps, if it be an incandescent light circuit, 
by a small margin, and only to exceed the combined current 
consuming capacity of the motors, if it be a motor feeder 
circuit, by 25 per cent. 

Should the current flow increase, by reason of short cir- 
cuit, grounds or rise in voltage, by an amount sufficient to 
cause overload, the fuse wire would become hot quickly, 
and, its melting temperature being far below that which 
would injure a copper wire, the fuse will melt and stop all 
current flow before the wires of the circuit, or even the 
apparatus attached thereto could be injured. 

Assume, for example, a circuit of R — C wire rated at six 
amperes, with a sufficient number of incandescent lamps 
attached thereto to consume a total of five amperes. We 
would insert between the terminals in Fig. 19 fuses having 
a capacity of 5 amperes. As a matter of fact our 5-ampere 
fuses would actually carry more than that, because fuses 
are designed and intended to carry 10 per cent, more than 
their rated capacity, in order to allow for ordinary fluctu- 
ations in voltage. 

Fuses protect a circuit because they melt at a temperature 
far below that necessary to injure copper wires. 

Fuses protect the apparatus because they heat very 
quickly under overload, melt and stop all current flow before 
sufficient time has elapsed to injure lamps or motors. 

Not only is the fuse a safeguard in the way we have de- 
scribed, but it is also an insurance against the operation of 
a faulty circuit. Because if the attempt is made to install a 
new fuse before the trouble which caused the blowing of 
the old one has been remedied, the trouble which blew 
(melted) the old fuse will also blow the new one. 

The foregoing is the theory of the fuse and an explana- 
tion < of its practical operation. In practice, however, raw 
fuse wire is now seldom employed, and never in a theatre, 
except in the form of "link" fuses, which are permitted, and 
even required by some cities for the fusing of projection 
circuits, but they are located in a fireproof projection room 
and must be placed inside an iron cabinet as well. 

The types of fuses with which the motion picture pro- 
jectionist is likely to come into contact, are the "plug" and 


the "cartridge," both of which forms are in general use in 
theatres. In fact they are the only fuses used in theatres, 
except as before noted with relation to the link fuse. 

In Fig. 20, A is a cartridge fuse having "ferrule" (see 
XX in figure) contacts, and B is a cartridge fuse having 
knife blade contact, the first named being permitted only 
on circuits carrying 60 amperes or less. C and D are re- 
spectively the receptacles for fuses A and B. 

CARTRIDGE FUSES.— A cartridge fuse consists of two 
terminals joined by a barrel constructed of insulating 

Figure 20. 

material. Inside this barrel is a conductor made of fuse 
metal, which connects the two terminals, and a small wire 
known as the "pilot" wire also connecting the terminals and 
passing under a round spot on the paper label attached to 
the fuse, as per illustration in Fig. 21. An air chamber is 
used in some fuses, the idea being that the heat conduction 
through the confined area being slow, the temperature of 
that part of the fuse will rise rapidly, and always in the 
same ratio, which is persumed to establish a practically 
constant point of blowing. Except in the air chamber the 
fuse wire is surrounded by a powdered, non-conducting sub- 
stance, designed to instantly break the arc when the fuse 
blows. On a paper label pasted on the outside of the 
barrel of the fuse is a small round spot under which the 
pilot wire passes. When the fuse blows the arc formed 
when the pilot-wire melts is presumed to char the paper, 
and thus turn the spot brown or black, although it does not 
always perform its duty in this respect. Table No. 4, which 



is taken verbatim from the National Electric Code of Fire 
Underwriters, gives the essential underwriters' require- 
ments in the matter of dimensions for cartridge fuses. The 
underwriters require that the contacts have a certain 

Figure 21. 

minimum area, that the paper barrels have a certain mini- 
mum length and diameter, and that the fuses have a certain 
definite length over all for a given voltage and amperage. 

PLUG FUSES.— Plug fuses are freqently used to protect 
theatre incandescent circuits. A plug fuse consists of a 
receptacle similar to that illustrated at B, Fig. 22, and a 
porcelain "plug," with a cap, usually of brass, as per A, 
Fig. 22, the brass cap which may or may not have a mica 
window through which one is presumed to view the fuse 
and ascertain its condition. Usually, however, this is not a 
practical thing to do, and the condition of the fuse may only 

Figure 22. 


be definitely ascertained by testing as hereinafter directed. 
C, Fig. 22, shows the porcelain base of the fuse plug with 
the cap off and the fuse in place. D, Fig. 22, is a special 
form of plug fuse to be used on amperage between 35 and 60, 
plug fuses in their regular form not being made in excess of 
35 amperes capacity. They are not made in any form for 
capacity in excess of 60 amperes. Plug fuses may be used 
for any kind of work desired, up to the limit of their 
capacity. They are just as safe, and somewhat cheaper than 
cartridge fuses. 

LINK FUSES.— The link fuse, illustrated in Fig. 23, is 
specified for use on projection circuits by the authorities of 

Figure 23. 

New York City and by some other municipalities. This is 
by reason of the fact that it is difficult to "boost" a link fuse 
without the inspector being able to instantly detect the 

Where link fuses are used for projection circuit pro- 
tection they must be located in a metal cabinet having a 
self-closing door, and this cabinet must itself be located 
inside the projection room. 

The link fuse consists of copper terminals A A, Fig, 23, 
and fuse wire B, terminals A A being clamped under the 
terminal screws of a link fuse block. 

BOOSTING FUSES.— Boosting a fuse consists in in- 
creasing its capacity by means of a small copper wire, or in 
case of a plug fuse a copper coin or something similar. 
Such practice is reprehensible in the extreme. It is, in fact, 
next door to criminal. A fuse is for the protection of wires 
and apparatus, and a boosted fuse no longer serves its 
purpose. It leaves the circuit without any protection at all, 
under which condition there is a possibility of serious 
damage to the apparatus, and of heating the circuit wires 
to the point where they will set the building on fire, or be 
fused by the heat. 

With both cartridge and plug fuses it is possible for a 
projectionist possessed of more cunning than good sense to 



Porm 1. CARTRIDGE FUSE— Ferrule Contact. 


Not over 

Not over 















8 * 

























Porm 2. CARTRIDGE PUSE— Knife Blade Con- 


Diameter of 

Ferrules or 


of Terminal 






Min Lenjrth 

of Ferrules 


or of Termi- 


nal Blades 


outside of 






. % 






























i % 
















Number Four. 


increase the capacity of his fuses almost indefinitely by 
"boosting," and such a trick could only be detected by a 
very close inspection. With the link fuse, however, this 
cannot be done so readily. Hence link fuses are recom- 
mended, under the conditions of installation named, for pro- 
jection circuits. 

Any projectionist or other person caught boosting fuses 
should be instantly discharged, and if he holds a license it 
should be suspended for a first offense and revoked for a 

Never fuse above the rated capacity of the wires of the 

Never fuse an incandescent lamp circuit above the com- 
bined amperage capacity of its lamps. 

Never fuse a motor circuit above the rated capacity of the 
wires, or more than 25 per cent, in excess of the rated 
capacity of the motor or motors. 

Underwriters* rules allow the fusing of a motor circuit to 
25 per cent, above the capacity of the motor or motors 
attached thereto, provided, of course, the wires be large 
enough to accommodate the capacity of the motors plus the 
25 per cent, overload. 

It is physically possible to refill both cartridge and plug 
fuses, but it does not pay to do so, except in the case of 
special fuses made to be refilled. 

THROW OLD FUSES AWAY unless they be of the "re- 
filling" sort. Fuses which have blown have absolutely no 
commercial value. They should be thrown away imme- 
diately, else they may get mixed with the good fuses, with 
consequent possibility of vexatious delay — and such delays 
usually occur just at the worst possible time. If you keep 
your old fuses, and get them mixed with the good ones, for 
such delays you have no one but yourself to blame. 

circuit wires are usually of size amply capable of carrying 
considerably more current than will ordinarily be used. 
Both the lamp and the wires of the circuit offer no chance 
of damage through a considerable temporary overload. It 
not only is a nuisance, but also impractical to have pro- 
jection room fuses constantly blowing, and since the re- 
sistance oi a projection arc lamp, especially if it be hand 
fed, is a highly variable quantity, the current flow will 
under any conditions vary considerably. We would there- 
fore recommend for projection circuits the following, with 



the understanding that the ordinary current flow at the 
arc is what is referred to under the heading "normal 
amperage." Of course if the fusing is only done on the 
primary of a transformer (Compensarc Inductor Econo- 
mizer, etc.) then due allowance must be made, as is set 
forth, see bottom of this page and next page. 

Note — Fuses cannot be had in all the sizes named. This 
acts to limit the application of table No. 5. 

Necessary Necessary 

size R. C. size asbestos 

Normal Arc 

portion of < 

covered pon 


Fuse to 

circuit wires 

circuit wj 




























































No. 5. 

Projection circuit fusing table where rheostats are used 
for resistance. 

Note — Asbestos covered stranded wires are not available 
in a size larger than No. 4. We have, therefore, given the 
necessary sizes for doubling. "8 + 8" means two No. 8 wires 
instead of the necessary No. 3 for 100 amperes, and a 6 and 
an 8 for 110 amperes. 

Explanation — Wires must be large enough to accommo- 
date the amperage capacity of the fuses without overload- 
ing. That portion of the circuit which is asbestos covered 
wire may be treated as weatherproof in this respect. See 
wire capacity table, page 70. 

ATOR. — Fusing the projection circuit where a motor gener- 
ator rotary converter or mercury arc rectifiers is used is a 
simple matter. Ordinarily there should be fuses on both the 
motor and the generator side — on the intake and the output. 
Ascertain the amperage at the arc under normal conditions, 
and add about 20 per cent, to that amount, which will give 
the correct size for your fuses on the generator, or output 


side. The arc will, of course, be D. C, and for the purpose 
of figuring, the table on page 400 should be used. If, for in- 
stance, we have a 60-volt arc, the result of the arc amperage 
multiplied by 60 will give the arc wattage, which, divided by 
the voltage of the supply lines will give the intake amperage, 
or would give it if the machine had 100 per cent, efficiency. 
Few such machines, however, have more than a 65 per cent, 
efficiency; therefore, to the result so obtained, about 35 per 
cent, must be added in order to get the actual intake amper- 
age thus: Assuming a line voltage of 110 and an arc wattage 
of 4200 (70 amperes with a 60 volt arc), then 4200 watts -*- 
110 volts = 38+ amperes, which, with the addition of 35 per 
cent, for losses in the machine itself, would be the amperage 
taken from the line. 35 per cent, of 38 amperes is 13 3/10th 
amperes, and, disregarding fractions, 13 + 38 = 51, which 
would be the total amperage taken from the line. 

Of course, in the foregoing we are merely showing you how 
the thing is done. In order to get anything like an accurate 
result it would be necessary to measure your arc voltage with 
a voltmeter, and to measure the efficiency of your motor 
generator and the latter is not a thing the ordinary projection- 
ist is equipped to do with any large degree of accuracy. 

Assuming, however, that we find the intake amperage to 
be 51, we would install 55 ampere fuses on the intake line, to 
protect the motor, and since the generator output is 70 
amperes it would only be necessary that we install seventy 
ampere fuses on the generator side, unless it were necessary 
to overload the machine at change-over, under which condi- 
tion we would necessarily fuse for the overload, whatever it 
might be. 

The foregoing is, however, qualified by the fact that if the 
generator is of higher voltage than the arc, then the arc 
amperage must be multiplied by the voltage of the generator 
instead of the voltage of the arc, since resistance will have 
to be used to cut down the voltage of the generator to the 
arc voltage, and voltage consumed in resistance counts just 
the same as that used in operating the arc. 

SUPPLY OF FUSES.— The careful man will always keep 
plenty of fuses on hand. One never can tell when a fuse will 
blow. Sometimes an epidemic of fuse blowing occurs. It is 
bad to be caught without fuses, and the only insurance 
against it is an ample stock of surplus fuses. 

In case you do get caught without fuses it is possible to 
protect the circuit reasonably well for a temporary period 


with one fuse, bridging the other fuse terminals with a 
copper wire. This, however, may only be tolerated as a 
strictly temporary expedient in case of emergency, until 
proper fuses can be procured. Emergencies of this kind 
should never occur. 

A better emergency substitute is to make a fuse of copper 
wire. While such a fuse would be unreliable to a consider- 
able extent, and from every viewpoint objectionable, still it 
may be used temporarily in an emergency to bridge one fuse 
contact, provided the other fuse be in good condition. We 
therefore give the fusing point of small copper wires. 

Fusing Point of Copper Wires. 

American (B. & S.) Wire Gauge Fusing Current in Amperes 

30 ' 10 

28 15 

26 20 

25 25 

24 30 

22 40 

21 50 

20 60 

19 70 

18 80 

17 100 

16 120 

15 140 

14 160 

13 200 

By combining strands of an asbestos covered wire, which 
usually are either No. 30 or 31, a fuse of almost any desired 
capacity may be had. Thus, five strands would be about 
right for 40 amperes. 

WHEN FUSES BLOW.— When a fuse blows and the new 
one you install also immediately blows, it is conclusive proof 
that there is heavy overload, most likely due to a "short" or 
"ground," and the circuit must be left dead until the trouble 
is located. See testing for grounds, page 356. 

A rise in voltage will operate to force more current through 
the lamps and motors, thus causing an increase in amperage 
which may blow the fuses. This condition will make itself 
evident by the incandescent lamps burning above candle 

FUSE CONTACTS.— Should a fuse blow and the new one 
installed also blow, but only after a more or less extended 
time, it is likely the trouble will be found in the fuse con- 

Examine the fuse contacts carefully, since loose or d*rty 



contacts will generate heat, which may be sufficient to cause 
the trouble, especially if the fuses are working near their 

TESTING FUSES.— Often when fuses blow it is difficult 
to tell which one of the two it is. We would therefore recom- 
mend the installation, at some convenient point, of a fuse 
tester made as per Fig. 24, in which A and B are the wires 
of any circuit that is always "alive," preferably the main 
feeders ahead of the switchboard fuses. If you attach at 
this point and the house is fed by a three-wire system, be 

Figure 24. 

sure to attach to one outside wire and the central or neu- 
tral wire, else you will have 220 volts on your tester instead 
of 110. D is an ordinary cartridge fuse receptacle, which 
must be ferrule or knife blade, according to the type of fuses 
used. E is a plug fuse receptacle. C is a receptacle and an 
incandescent lamp of the voltage of your current. When you 
put a fuse in either of the receptacles and lamp C does not 
light the fuse is worthless, and should be thrown away. 

FUSE MARKINGS. — Cartridge fuse voltage and amperage 
rating are usually found marked on the paper label of their 
barrel. Plug fuses have their ratings stamped on the brass 
cap or the center contact and link fuses have, or should have 
their rating stamped on one of the copper contacts. 



WHERE FUSES ARE INSTALLED.— In general, fuses 
are installed as follows : (A) Main service fuses, located 
ahead (on the street side) of the main house switch. These 
fuses carry all the current used in the theatre except the 
exit and other lights ordinarily left burning during the per- 
formance. Circuits carrying these latter, called emergency 
lights, should be attached to the feed wires ahead (on the 
street side) of everything else, and have service fuses of 
their own. See "Fusing Emergency Light Circuits," page 120. 

Note — In some theatres the stage is fed by a separate set 
of feeders coming from the street mains, in which case this 
circuit will, of course, have main fuses of its own. (B) Fuses, 
usually on the main house switchboard, protecting the pro- 
jection room service circuit. (C) Fuses on the main house 
switchboard protecting the service wires for the stage, if the 
stage takes its current through the main switchboard, as is 
usually the case. (D) Main fuses in the projection room 
which protect all projection room circuits; also individual 
service fuses on every separate projector arc circuit and 
projection room motor and incandescent circuit. (E) Fuses, 
ordinarily located on the main house switchboard, for each 
individual auditorium incandescent and motor circuit. (F) 
Fuses on the stage switchboard for each individual circuit, 
as well as main fuses carrying all stage circuits. (G) Fuses, 
usually located in the box office, carrying the entire emer- 
gency light system, as well as fuses for each individual 
emergency light circuit. (H) Fuses for each individual 
emergency light, particularly in the case of exit lights. (1) 
Fuses must be installed wherever a change in size (diameter) 
of wire occurs. 

Figure 2b 



fuses for emergency light circuits should be located ahead 
(on the street side) of everything else, including the main 
house switch. In addition to this, every separate emergency 
circuit must have fuses of its own, and still in addition to this 
it is an excellent plan to fuse each individual emergency 
light, especially the exit sign lamps, with one-ampere fuses. 
This latter is by reason of the fact that if trouble develops 
in a lamp, it will then blow only its own fuse, without dis- 
turbing the other emergency lights, where otherwise it would, 
or at least might put an entire circuit, or possibly even the 
entire emergency light system, out of business. 

Every circuit, no matter how large or small it may be, must 
be protected by its own individual fuses, in addition to the 
main fuses carrying all circuits. 

ing of projection circuit fuses is a very annoying thing, since 
it stops the show and causes delay while new fuses are being 
installed. It does not necessarily follow that there is any- 
thing wrong because a projection arc lamp circuit fuse blows, 
particularly if the circuit is not fused much above the amper- 
age being used. By installing two sets of projection circuit 
fuses as per Fig. 25, delays of this kind are avoided. When 
a fuse blows the projectionist has only to throw over the 
D.P.D.T. switch to cut in a new set of fuses, and unless 
there be something wrong with the circuit itself no appre- 
ciable delay will occur. 



Wire Terminals and Wire 

IN the course of his duty it is necessary that the projec- 
tionist on the road with a traveling show, or the pro- 
jectionist of the small-town theatre do more or less wire 
work, or at least that he have an understanding of many 
things connected with wire work. 

TERMINAL LUGS.— Every wire should have a terminal 
lug, and except in cases where lugs will be subjected to heat, 
as in the projector lamp-house or at the rheostat, they 
should be soldered to the wire. Terminal lugs come in a 
number of forms, two of which are illustrated at E F, Fig. 26. 
In soldering a lug to the wire, proceed as follows : First 
measure the depth of the socket in the lug and cut off just 
sufficient of the insulation of the wire to allow its end to 
reach the bottom of the hole in the lug. Make the cut a 
square one, but be very careful not to cut clear through 
against the wire, because if you do the edge of your knife 
will most likely cut a tiny ring in the outside of the wire, 
which v will to a large extent act the same as the cut made 
on a glass by a diamond. A knife cut on the outer surface 
of a copper wire weakens it greatly, so be careful. 

Having removed the insulation, as per B, Fig. 26, scrape 
the bare wire-end perfectly clean. This latter is important, 
since otherwise the solder cannot make perfect contact be- 
tween the wire and the lug. Next, first having made sure 
the inside of the socket of the lug is perfectly clean, hold 
the lug in the flame of a blow torch, or some other heat 
source, and melt sufficient solder into it to fill the hole about 
half full. Don't get the lug too hot, but just hot enough to 
make the solder thoroughly liquid. Now, first having rub- 
bed on the bare wire end a little paste soldering flux, shove 
it down into the solder in the lug and hold it then until the 
solder cools. 

CAUTION: Do not shove the wire into the lug with a 
quick push. If you do, the hot metal will probably squirt 



out and you may get badly burned. If the weather is cold 
it will be well to warm the end of the wire a little before 
shoving it into the lug. If these directions are followed you 
should have a mechanically strong and a perfect electrical 

In attaching terminal lugs to binding posts, be very sure 
that both lug and the binding post are perfectly clean. If 
they are not, scour them with a bit of sandpaper or emery 
cloth, or scrape them clean with a knife blade. It is par- 
ticularly important that a copper wire be perfectly clean 
when it is joined directly to a binding post without a lug, 
since often a thin coating of oxidization will cover the metal, 
and this coating, while it is usually thin enough to be in- 
visible, offers high resistance. A wire attached to a binding 
post by means of a properly soldered lug should offer no 
more resistance than the same length of the wire itself would 
offer, but if the connection be improperly made it may offer 
considerable resistance — perhaps enough to make the joint 
hot. This will itself operate to still further increase the 
trouble, since heat increases the resistance of metals. 

The resistance of one imperfect joint might or might not 
amount to much, but that of several would waste many 
dollars worth of electrical energy in the course of a year, 
and it is well to remember that the meter registers all energy 
consumed, whether it be used in overcoming the useless re- 
sistance of poorly made joints, or in the production of light. 



Figure 26. 


WIRE SPLICES.— In Fig. 26 several correct methods of 
making splices are illustrated. First the insulation must be re- 
moved from the ends of the two wires to be joined for a dis- 
tance of from 2 to 3 inches, according to the size of the wire. 

The insulation should be whittled away just as you would 
whittle a lead pencil in sharpening it. Do not cut the insula- 
tion square off by running the knife blade around the wire. 
It makes a neat -looking jub, but the knife blade is apt to cut 
a slight ring around the wire, which, as before set forth, acts 
much as does the scratching of the surface of glass with a 
diamond, causing the wire to break very easily at that point. 
The correct method of trimming off the insulation for the 
making of a splace is shown at A, Fig. 26, and the wrong way 
is illustrated at B. 

After removing the insulation, the wire ends must be thor- 
oughly cleaned, until they shine. This may be done with 
emery or sandpaper, or by scraping with a knife blade. Un- 
less the wire be made perfectly clean there will not be good 
electrical contact. After being thoroughly cleaned the wire 
ends must be twisted together tightly, as at I, Fig. 26, after 
which the joint must be soldered. 

Underwriter's rules provide that a wire splice must be 
made both mechanically and electrically perfect before sol- 

To solder, wet the metal thoroughly with a soldering fluid 
or its equivalent, which latter may be had from electrical 
dealers in stick or paste form. After thoroughly covering 
the joint with the fluid, or rubbing paste or stick flux on, 
hold both the wire and the end of a piece of wire solder in 
the flame of a blow torch until the solder melts and runs all 
through the joint. 

CAUTION. — Care must be observed not to get the wire too 
hot, especially with small wire, since too much heat causes 
injury to the copper, reducing both its tensile strength and 
carrying capacity. If too much heat is used the solder will 
run through and out of the joint. If the soldering be proper- 
ly done the joint will have greater mechanical strength and 
carrying capacity than the wire itself. 

After soldering, the wire must be wrapped with insulating 
tape to the depth of the original insulation, the first layer 
of which should be what is known as rubber tape, with an 
outer covering of ordinary adhesive cloth tape. 

What is perhaps the best method of making a splice in 


asbestos covered stranded wire is illustrated at C, Fig. 26, ex- 
cept that the strands should be divided into about six groups. 
Under some conditions a wire connector similar to D, Fig. 
26, may be used, but wire connectors such as this cannot be 
used to join the ends of stranded asbestos covered wire, or 
other stranded wire, unless the ends of the wire be first run 
full of solder, thus binding the strands together in a solid 

SOLDER FLUX. — An excellent soldering fluid is composed 
of the following. The mixture may be compounded by any 
druggist. It works well on either copper or tin. 

Saturated solution of zinc chloride 5 parts 

Alcohol 4 parts 

Glycerine 1 part 

ber of terminal lugs designed to be used without solder 
where the service is such that a soldered terminal would be 
impractical, as in the case of old style projection arc lamps, 
rheostat binding posts, etc. These lugs make contact with 
the wires by means of compression. In attaching them be 
certain the metal of both the wire and lug is perfectly clean. 
They were in common use before projector manufacturers 
began equipping their arc lamps with clamps to receive the 
wire in such form that terminals are no longer required for 
that service. They may still be had from supply dealers, and 
are suitable for connecting the wires to rheostat binding 
posts, although most modern rheostats will permit of the 
use of a soldered lug, especially if a rather hard solder be 
used. The soldered terminal is much the best. 








THERE is a law which deals with light action with rela- 
tion to its intensity at different distances from its 
source as follows : 
"Light intensity decreases inversely as the square of the 
distance from its source." 

In Fig. 27A, A, B and C represent screens held before an 
open light source at different distances therefrom. Remem- 
bering that light travels in straight lines, it is readily seen 
that screen A would receive all the rays falling within the 
two black, diverging lines, and that screens B and C could 
only receive the relative portions as indicated. See further 
explanation accompanying Fig. 36 H, page 162, which explains 
the action and effect of the inverse square law, as above 
quoted, as applied to practical projection. 

OPTICAL TRAIN.— The optical train of the motion picture 
projector is made up of two entirely separate lens com- 



^*n. LBNS *r PO/MT £ ///TWfRf 
A My op utnr m PPrH wma bs rwf 
00TTE3 Lint 

Figure 27. 



binations, optically so joined that they become, in effect, a 
compound lens system. 

element of the system is the condenser, the function of 
which is to receive the diverging rays from the light source, 
refract and converge them to what is known as the "spot" 
at the projector aperture. Put another way, the office of 
the condenser of a motion picture projector is to direct 
upon the projector aperture the greatest possible amount 
of the total available light. 

second element of the system is the projection lens. Its 
function is to receive the diverging light rays carrying the 
film image, and to refract and focus them at the screen in 
an enlarged, reversed image of the film picture. 

TECHNICAL TERMS.— There are many technical terms 
used in connection with lenses and optics, but we believe; 
that, insofar as concerns the projectionist, only a few are of 
any considerable importance. 

PRINCIPAL AXIS.— The principal axis of a lens is an 
imaginary line which passes exactly through the center of 

its diameter, and is exact- 
ly perpendicular to its 
plane, remembering that 
in optics "perpendicular to" 
means at right angles to. 
In Fig. 27 point F is the 
center of curvature of the 
surface of the lens far- 
thest away from it. Line 
F A is the principal axis 
of the lens, because it is 
perpendicular to line B, at 
the center of the diameter 
of the lens and line B is 
the plane of the lens. It 
will thus be seen that if 
line F A were a ray of 
light it would not be re- 
fracted, because it would 
meet both surfaces of 
the glass exactly at 
right angles, or, in 

Figure 27 A. 


optical terms, would be perpendicular to both surfaces of 
the lens, hence would pass straight through. 

Let us also examine line C, which, too, will meet one sur- 
face of the lens, the one farthest from point F, exactly at 
right angles, because any line drawn from a point of curva- 
ture will be exactly perpendicular to the surface at the 
point it meets it. In Fig. 27 line D is intended to represent the 
surface of the lens exactly at the point line C meets it. 

But, as a matter of fact, if line C be a ray of light it will 
not continue straight through as shown, but will be re- 
fracted by the first surface, because it meets the surface at 
an angle, and will follow the path indicated by the dotted 
line. Likewise line G would be perpendicular to the lens 
surface at E, if it were continued straight through. But it 
is refracted by the first surface, and refracted very much 
more than line C because it meets the glass at a much 
greater angle. 

Neither line C or line G is a principal axis, because neither 
is perpendicular to the plane of the lens as represented by 
line B. 

CONJUGATE FOCI.— Conjugate foci is a term having 
reference to two points, one being the distance of the optical 
center of the lens from a light source, or from an object, 
and the other the distance from the optical center of the 
lens to the point at which the rays from the light source 
or object are focused into an image. The conjugate foci 
points are shown in Fig. 30, in which object X (candle) is 
one point and the image, Y, the other. Altering the dis- 
tance of the object from the lens automatically alters the 
distance of the image. If the candle (Fig. 30) be moved 
nearer the lens, then image Y will automatically be re- 
moved further away, and vice versa. In a projector optical 
train the conjugate foci points of the condenser are the 
light source and the image of it which is formed near the 
"spot," while the conjugate foci points of the projection lens 
are the film and the screen. 

REFRACTION. — The action of lenses is based upon the 

Light rays travel in straight lines in any transparent 
medium of even density, but are bent or refracted while 
passing from a medium of one density to a medium of 
another density, provided the rays enter the second medium 
at an angle to its surface. 



It therefore follows that, glass and air being of different 
density, if rays of light pass from one to the other at an 
angle to the surface of either medium, they (the rays) will 
be bent, or "refracted." Concerning this, "Optic Projection," 
page 576 says : 

"The amount of bending depends upon two conditions ; 

(1) The greater the angle of incidence of the light, that is, 
the further from the perpendicular or normal that the light 
strikes the surface, the greater will be the bending upon 
entering the second medium. And this increase is not simply 
with the increase of the angle of incidence but propor- 
tionally greater, that is, in accordance with the law of sines. 

(2) The bending depends also upon the difference of den- 
sity of the two transparent media. If the difference is great, 
the refraction will be great, and if the difference of density 
is small, the refraction will be proportionally small." 

It is not the purpose of this work to instruct in optics, 
except insofar as we feel is necessary to give the pro- 
jectionist a broad understanding of lens action. We do hot 
expect to enable him to determine the refractive index of 
glass, but we do expect to give him a comprehensive under- 
standing of how and why a lens refracts rays of light, and 
focuses light rays to an image. 


Figure 28. 

In Fig. 28 we see three rays of light incident upon a simple 
bi-convex lens. Ray A strikes the lens surface at a com- 
paratively heavy angle, hence is bent (refracted) at a heavy 
angle. Ray B strikes the surface of the lens at a less angle, 
hence is refracted in less amount. Ray C strikes the glass 
perpendicular to its surface hence is not refracted at all, 
but passes straight through. 


Since air and glass are always the mediums for trans- 
mission of light, insofar as concerns the projectionist, and 
the density of glass varies but little, we may roughly as- 
sume that the amount of refraction the rays will receive 
from a lens will be dependent almost entirely upon the 
angle at which it encounters the surface of the glass, either 
in entering or leaving the lens. 

Those who wish to know why the rays are bent under the 
conditions named will find the explanation in any good 
work on physics. It seems hardly worth the space to set 
forth the matter here. 

ANGLE OF INCIDENCE.— The angle of incidence is the 
angle the entering rays makes with a line perpendicular to 
the surface of the medium. 

ANGLE OF REFRACTION.— The angle of refraction is 
the angle a ray makes with a line perpendicular to the surface 
of the medium after leaving it. 

WORKING DISTANCE.— See page 49. 

EQUIVALENT FOCUS (E. F.).— A term applicable to 
compound lenses consisting of two or more individual ele- 
ments, as in the case of the projection lens. It means that 
the combination will possess the same power of reduction 
or magnification possessed by a single, simple lens having 
the same focal length as the equivalent focus of the com- 
bination. For instance : If your projection lens is a 4.5 
inch equivalent focus (E. F.) then it will, when working 
under the same conditions, project the same size picture 
that a single lens of 4.5 inch focus would project. Equiva- 
lent focus is of value to the projectionist in computing the 
focal length lens required to project a given size picture at 
a given distance. See page 155. 

SPHERICAL ABERRATION.— Spherical aberration is 
that quality of a simple lens which causes it to focus rays 
which pass through it at varying distances from its prin- 
cipal axis at different distances from its optic center. The 
professors Gage, in their book, "Optic Projection," define 
it as "the unequal bending of the light rays in different 
zones of a lens." This we do not regard as correct as to 
language (although what is meant is correct enough), be- 
cause naturally the rays will be bent unequally in different 
zones of a lens. Were this not so there could be no reduced 
or magnified image with parallel rays incident. 


Rays passing through the outer edge of a simple uncor- 
rected converging lens cross the principal axis of the lens 
at a point nearer its optic center than do rays passing 
through the lens nearer its principal axis. This is illustrated 
in Fig. 29. 

CHROMATIC ABERRATION.— Avoiding a technical 
definition, chromatic aberration is that quality of a lens which 

Figure 29. 

causes it to separate white light more or less into its primary 
colors. It is a quality of simple lens action which causes it 
to focus different color waves at different points or distances. 
Different kinds of glass have different characteristics in this 
respect, and by combining different glasses and curves 
chromatic aberration is corrected. See Lens correction, 
page 132. 

It is in order to accomplish corrections of spherical and 
chromatic aberration, and other faults that several lenses 
are used in making up a projection lens. The different kinds 
of glass and different kinds of lenses combine to make the 
desired corrections. The condenser is uncorrected, hence it 
has spherical and chromatic aberration, as well as all the 
other faults inherent in uncorrected lenses. The chromatic 
aberration of the condenser causes the colored light which 
surrounds the spot, and its spherical aberration carries some 
of the color down into the center of the spot, thus injuring 
the brilliancy of the light projected to the screen. 

PROJECTION ANGLE.— See pages 20 and 255. 


STANDARD FILM.— Shall be one and one-third inches 
wide, shall carry one picture to each four perforations, and 
sixteen pictures to each foot of film 


CONDENSER.— See pages 25 and 164. 
LIGHT BEAM.— See page 32. 
Light RAY.— See page 32. 
WORKING DISTANCE.— See page 49. 

THE MECHANICS OF LENSES.— In order to understand 
the action of. light through lenses it is necessary that the 
student get the "viewpoint." This is a very difficult thing to 
do, mainly by reason of the fact that it is quite possible to 
view light action through lenses in more than one way, 
although regardless of which one of the possible views we 
may take the ultimate result is essentially the same. 

From the optical viewpoint each tiny pin point of the 
surface of a lens presents an entirely separate proposition 
from every other pin point, because of the fact that it pre- 
sents a different angle to the light, thus producing refraction 
slightly different from that of the point next adjoining it, 
and different from every other portion of the surface of the 

One fundamental fact the student should get firmly fixed 
in his mind is that : 

Once a light ray has entered a lens at an angle to its sur- 
face, and has thus received its initial bending, or refraction, 
it will, if the glass be of even density, travel in a perfectly 
straight line until it reaches the opposite surface of the lens, 
where it re-enters the air, and in so doing receives its second 
bending or refraction. 

We thus see that, except for the slight variation in density 
of glass, the action of a lens is dependent wholly upon its 
surfaces, which fact should impress us with the imperative 
necessity that lenses have surfaces which are optically true. 

If a light ray enters and leaves a lens at exactly right 
angles to both surfaces of the glass there will be no re- 
fraction and the ray will pass straight through, but if it 
strikes the glass at an angle the ray will be bent or re- 

Thus, let us assume a pin point of light located on the 
optical axis of a simple lens. Rays from the light source, of 
course, diverge in every direction. The ray meeting the lens 
surface at the optical axis will meet the surfaces of the glass, 
both entering and leaving, at precisely right angles to its 
surface. In optical language, the ray will be "perpendicular" 
to the surfaces of the lens, hence there will be no refraction. 


This particular ray will pass straight through, but the sur- 
face of the lens being curved at every point, the ray which 
strikes the lens ^th or even l/100th of an inch from the 
optical axis of the lens will meet the glass at a slight angle. 
Hence the ray will be refracted, and the amount of refraction 
will be partly dependent upon the distance of the light 
source from the lens, since the distance of the light source 
from the lens of course controls the angle of the rays thereto. 
The same is true of the ray that meets the lens half an inch 
from its optical axis. In this case the ray will be refracted 
more than the one which meets the lens nearer to its 
optical axis. 

A very understandable illustration is found in the con- 
denser. Suppose the arc to be a pin point of light located 
3.5 inches from the surface of a 4.5 inch diameter lens. The 
ray which strikes the lens half an inch from its optical axis 
will not strike the glass at much of an angle, hence its initial 
refraction or bending will be slight, but the ray which strikes 
the lens near its outer edge will meet the glass at a heavy 
angle, hence its initial refraction will, by comparison, be very 
great. (See figure 28, page 128.) 

LENS CORRECTION.— All uncorrected lenses have both 
spherical and chromatic aberration. By means of a com- 
bination of different kinds of glass and positive and negative 
curvatures it is possible to correct lenses for both spherical 

Figure 30. 

and chromatic aberration. As a matter of fact, projection 
lenses are thus corrected. In this correction what is known 
as crown and flint glass are used. See Chromatic Aberration, 
page 130. 



Those who wish further enlightenment on this matter, which 
has entirely to do with practical optical work, will find 
lengthy treatises on the subject in various optical works, 
which may be consulted at the public library. Also see "Optic 
Projection," page 581 and Fig. 324. 

IMAGE FORMATION.— Assuming the lens in Fig. 30 to be 
free from spherical aberration, all rays emanating from any 
given point on light source X and striking the surface of the 
lens will be refracted in such manner that they will again 
meet at point Y, these two points being called the "conjugate 
foci" points of the lens. 

If light source X be located nearer the surface of the lens, 
point Y will automatically be moved farther away from the 
surface of the lens, and if light source X be placed near 
enough to the lens, point Y will finally be lost and the rays 
will leave the lens in parallel, or even in diverging lines. 

On the other hand, if light source X be moved farther away 
from the lens, point Y will automatically be brought closer 

Figure 31. 

to its surface. It is this law of optics which is brought into 
operation when the film image is focused on the screen by 
adjusting the projection lens — moving it nearer to or farther 



away from the film, which is, of course, exactly the same 
thing as moving the film itself nearer to or farther away 
from the lens. Assuming the lens shown in Fig. 30 to 
represent the projection lens, X to represent the film, and Y 
the screen, if the lens be , too far from, or too close to X, 
then the Y conjugate foci point will not be at point Y, and 
the rays will not meet or focus there, in explanation of which 
see Fig 36D. 

FOCAL LENGTH.— The focal length of a simple lens is 
the distance from its optical center to the image, when the 
image is in sharp focus and the object sufficiently distant to 
cause the lens to receive parallel rays of light. The focal 
length of a lens is determined by the curvature of its surfaces 
and the refractive index of the glass from which it is made. 

In Fig. 31, we see two lenses, one of which, A, has slight 
curvature, while the other, B, has rather heavy curvature. 
By reason of its slight curvature, the refractive power of the 
lens shown at A is not so great as is that of the lens shown 
at B. It will thus be seen that the heavier the curvature of 
the surface of lenses of this type, the shorter is their focal 
length. This is by reason of the fact that the heavier the 
curvature the greater will be the angle at which parallel rays 
of light will strike the glass, both on entering and leaving 
the lens, hence the greater the amount of refraction 
the rays will receive, and the nearer the lens they will be 
brought to an image forming focus. With the exception of the 
parabolic converging condenser lens the surfaces of all 

lenses used for pro- 
jection work, both 
condenser and ob- 
jective, are always 
the section of the 
surface of a true 


In Fig. 32 we are 
enabled to see how 
the curvature of an 
ordinary lens is de- 
termined. Looking 
at the measurements 
we see that the out- 
Figure 32. er circle is 7 ^ in " 



ches in diameter and the inner circle fr/2 inches in diameter. 
Let us imagine the outer circle to represent the circumference 
of a glass ball, that we halve it and polish the flat sides of the 
discs of glass thus produced. We would then have two l l / 2 
inch diameter, l l / 2 inch focal length plano-convex lenses. 
If, on the other hand, we saw off a section 4^ inches in 

diameter from the 
surface of the ball, 
and polish the flat 
side, we will have a 
$y 2 inch diameter iy 2 
inch focal length 
plano-convex lens. If 
the same thing be 
done with a glass 
ball 6y 2 inches in 
diameter the same re- 
sult would be had, 
except that in halving 
the ball we would get 
two 6y 2 inch diam- 
eter, 6y 2 inch focal 
length lenses, and by- 
cutting off a section 
A l / 2 inches in diameter we would have a Ay 2 inch diameter 
6^2 inch focal length plano-convex lens. 

It will thus be seen that with a plano-convex lens the con- 
vex surface will always have the curvature of a circle, the 
diameter of which is equal to the focal length of the lens. It 
will also be noticed that the larger the diameter of such a 
lens the thicker it will be. 

The method of designing a meniscus lens is indicated by- 
Fig. 33, in which the smaller circle represents the circle de- 
termining the outer or convex curvature of the lens, as in 
Fig. 32. But instead of a piano surface on the opposite side, 
as in the case of the plano-convex, the meniscus has a con- 
cave surface. We therefore determine the degree of curva- 
ture for the concave surface by means of the larger circle 
shown in Fig. 33. 

The designing of curves for a meniscus lens is an optical 
problem, entirely too involved for any except a man trained 
in such work. 

FOCUS. — At the risk of repetition we desire to impress 
upon the mind of the student the fact that unless the object 

Figure 33. 



or light source be a pin point, a lens will not focus the light 
beam to a pin point. A lens is presumed to form an image, 
and an image always has area. The matter should be viewed 
thus: Rays emanate from each pin point of the object, or 
light source. (When projecting a light source, which is ex- 
actly what the condenser does, the light source becomes an 
"object," within the meaning of that term as here used.) The 
rays from the particular pin point, and all other pin points 
of the object or light source, are picked up by a considerable 
area of the lens, or perhaps by its entire area, and the rays 
from each particular pin point of the object are refocused to 
a corresponding, though perhaps a magnified or reduced 
point in the image. That is what is meant by "focus." It is 
illustrated in Fig. 30. 

A little study will enable the student projectionist to under- 
stand why he is unable to focus the condenser ray to a pin 
point. In the case of the condenser ray, the condenser is 
receiving rays from the entire area of the floor of the carbon 
crater, and is refocusing them to an image of the crater. By 
reason of the fact that the crater does not set square with 
the condenser (the lower part being farther away from the 
lens than the upper part, and the further fact that the un- 
corrected lenses used for con- 
densers set up spherical aberra- 
tion) the actual image of the 
crater will be focused over a con- 
siderable distance in the con- 
denser ray. Only a certain por- 
tion of the crater will therefore 
be in sharp focus at any one point 
in the condenser beam. Since the 
crater itself has considerable area, 
the image will have area, hence 
the beam cannot be focused to a point. 

PROJECTION^EENS.— A projection lens is in fact a com- 
bination of four lenses, two of which are located at the end 
of the lens tube nearest the film. These two lenses are sepa- 
rated by a spacing ring by some manufacturers, but not by 
others, and the combination is commonly referred to as the 
"back factor" of the projection lens. The two other lenses 
located at the opposite end of the lens tube are cemented 
into direct contact with each other by means of Canadian 
Balsam, and are commonly referred to as the "front factor" 
of the projection lens. The front factor, usually being 

Figure 34, 


cemented together, will, at a superficial glance, appear to 
be one thick lens. As a matter of fact, the combination con- 
sists of one negative lens and one double convex lens, as 
indicated in Fig. 34, which shows the detail of Gundlach- 
Manhattan lenses. 

It sometimes may happen that the balsam with which the 
front factor of the projection lens is cemented together 
will melt, thus causing the lens to have a sort of streaked 
appearance. Should this occur it will be necessary to send 
the entire lens back to the manufacturer to have the front 
factor recemented. It is possible to separate the lenses by 
heating them in hot water, but it will disturb the corrections 
of the lens. It is a job which can be properly done only by 
the manufacturer. 

KEYSTONE EFFECT.— Keystone effect and its accom- 
panying distortion cannot be removed or corrected by pro- 
jection lenses, specially ground or otherwise, it being the 
natural result of the difference in distance the rays at the 
top and bottom of the light beam must travel in order to 
reach the screen. See page 253. 

CLEANING LENSES.— It is essential to best results that 
the surfaces of the projection lens be kept scrupulously 
clean. Oil on the surface will cause very serious loss of 
definition in the picture, and even the faintest, almost im- 
perceptible finger mark will do the same thing, perhaps in 
lesser degree. An even distribution of an accumulation of 
dust particles from the air may not seriously affect sharpness 
of focus, but it sets up additional loss of light through reflec- 
tion. Hence from every viewpoint it is of great importance 
that the surfaces of the lenses be kept perfectly clean and 
highly polished. 

There are several patent preparations on the market for 
cleaning lenses, some of which are good. We believe, how- 
ever, that all the projectionist needs to keep his lenses in 
first class condition is half a pint of wood or denatured 
alcohol, diluted with half a pint of clean water. The com- 
bination fills a pint bottle, costs but a few cents, and if used 
economically lasts for a long time. 

Nothing but a perfectly clean chamois skin, or soft, per- 
fectly clean cotton cloths, such as old handkerchiefs, should 
be used for cleaning lenses. 

Lenses should be washed with a cloth saturated in the 

. alcohol solution, and then quickly polished while still wet. 

It should be a part of the daily duty of the projectionist to 


carefully examine the outer surfaces of his projection lenses 
and his condenser lenses, and to clean them if necessary. 
At regular periods the projection lens should be disassembled 
and the interior surfaces of the various lenses cleaned. In 
disassembling be very careful not to get the two rear lenses 
mixed because if they be wrongly replaced the corrections of 
the lens will be either ruined, or at least be so badly 
damaged that the lens will not give good definition. In 
replacing the rear combination get the side of heaviest 
curve toward the screen, and do not forget to replace the 
spacing ring, if there is one. 

The front (thick) lens combination should be placed in 
the mount, with its surface of greatest convex toward the 

The guiding rule in reassembling a projection lens is to 
place all lenses with their greatest convex toward the screen. 

While the lens is disassembled, carefully examine the in- 
terior of its barrel and if at any point the black paint has 
worn off so that the metal shows, retouch it with lamp 
black ground in Japan, thinned with turpentine. Lamp blaclj 
ground in Japan may be had at any first class paint store. 
It is known as coach painter's black. Unless the interior of 
the projection lens be kept coated with non-gloss black 
there will be reflected light projected, which may fall out- 
side the screen proper. Cases have been known where the 
interior of the lens barrel reflected so much light from the 
above cause that a circle of light surrounded the entire 

In reassembling, the lenses should be clamped snugly in 
their mounts, although it is advisable not to screw the ring 
down too tightly, remembering that in due time it must 
again be removed. 

Concerning this matter one lens manufacturer in an in- 
struction booklet on its lenses says : "In cleaning and 
assembling, first note whether the extension tube is at- 
tached to the front or rear end, so that you will replace it 
correctly. Clean both sides of the front combination, but 
do not remove it from its cell. To remove the retaining 
ring from the rear cell, press lightly on two sides of the 
ring with two fingers and unscrew it. Too much pressure 
will make it bind so that it will not turn. Clean inside sur- 
faces of the two lenses of the rear combination and replace 
in cell. Be careful that they are seated evenly, screw up 
the retaining ring just tightly enough to prevent them from 


moving, then clean the outside surfaces. Note that the rear 
lens is convex on both sides, the flatter side being the out- 
side rear surface. The retaining rings should face toward 
the center. Reversing the cells will disturb the correction." 

To remove grease or oil from the surface of a lens use a 
soft rag, free from grit, moistened with a little benzine. 

Be careful when screwing the parts together to avoid 
crossing threads and do not screw up any joint very tightly. 

Do not use a hard, sharp tool to remove retaining rings. 
It may slip and scratch the lens. 

In reassembling Gundlach-Manhattan lenses follow Fig. 
34, which is a cut supplied by that company. 

REPAIRING LENSES.— Should one lens of a projection 
lens be broken or injured, it may be replaced, but in order 
to do this it is absolutely necessary that the complete lens 
be returned to its maker, with the broken or injured mem- 
ber. The broken element has no value whatever, but unless 
it is sent, there is danger that the focal length of the lens 
combination will be changed in the replacement. 

Odd lenses or combinations of lenses have absolutely no 
value. They cannot be utilized by lens makers to build 
complete lenses, except at greater cost than a completely 
new lens. The difficulty arises in matching other lenses to 
the old ones. In cases where just one lens of a combina- 
tion is broken the manufacturer can ascertain the exact 
formula for the broken lens by measuring the fragments. 

not made with a free aperture (diameter of free opening) 
greater than half their focal length. For instance, a 4-inch 
focal length lens could not be made with a greater working 
aperture than 2 inches. In practice they are not even made 
that large. 

ORDERING LENSES. — In ordering projection lenses, give 
the following data: (A): Width of picture desired and dis- 
tance from the screen to projector aperture. If the lens is 
above center of the screen appreciably the latter measure- 
ment should be from midway between top and bottom of 
picture. (B) Make of projector. (C) Specify lenses with 
or without jackets. (D) If two lenses are ordered specify 
as to whether you want them matched or not. (E) If matched 
lenses are wanted and your projectors are new, it will not be 
necessary to give width of aperture, but if the projectors 
are quite old it will be best to have exact width of aperture, 


measured with a micrometer caliper. This is especially im- 
portant if one projector be new and the other quite old. 

RANGE OF FOCAL LENGTH.— Projection lenses from 2 
to 8-inch E. F. are carried in stock by manufacturers. 

MATCHING LENSES.— Do not attempt to order a lens 
to match one you have by giving the focal length on the lens 
barrel. These focal length markings cannot be depended 
upon for close work. They are usually only approximate. 

If you have a lens and want one to match it you must send 
the lens to the manufacturer to be measured, or else give 
the manufacturer the precise width of the projector aperture 
as measured by a micrometer caliper, the exact distance from 
aperture to screen and the precise width of the light upon 
the screen when there is no film in the projector. 

Even with these precautions you cannot be certain, be- 
cause an error involving the smallest fraction of an inch may 
result in unsatisfactory results, if the error be made at the 
right place. If there is a keystone, the distance from screen 
to aperture must be measured at the same distance from top 
or bottom of picture that the width of picture is measured. 
Better send the lens. It is the only sure way. 

When lenses are purchased in pairs they are usually 
matched by selecting lenses from stock. Suppose two lenses 
are wanted to project a picture of a certain width at a cer- 
tain distance. The dealer or lens maker finds by computa- 
tion that this requires a 5-inch E. F. lens. He tries one and 
finds it gives a picture a bit too large. He tries another and 
another until one is just right. He then, by a process of 
selection through tests, finds another which gives exactly 
the same size picture at that distance. 

Why is this, you ask? Why did not the 5-inch lens give 
what it was supposed to give? For the reason that lenses 
vary, and their markings are not accurate. A 4-inch E. F. 
projection lens may vary from 3.95 to 4.20 inches, according 
to the admission of one large lens concern. Exhibitors and 
projectionists should remember this when matched lenses are 

NOTICE. — The Gundlach Manhattan Optical Company 
marks the exact actual focal length of its projection lenses in 
the invoice sent to purchasers. This notation is in figures 
and in parenthesis. 

This bit of information should be carefully filed away in 
the projection room, because if a lens of that focal length 
is ordered later it will match the one you have. 



set the losses due to reflection from the polished surface of 
each lens at as much as 4 to 5 per cent., or a total of 8 to 
10 per cent, for each lens, or plate of glass. They further 
remark that if the surface be not perfectly clean, or per- 
fectly polished, the light loss may amount to as much as 15 
per cent, of the total from each surface. Some authorities 
place the loss even higher, giving a minimum value of 6 
per cent per surface. 

In this connection the following valuable data was con- 
tributed by Dr. Hermann Kellner of the Bausch & Lomb 
Optical Com- 
pany, manufac- 
turers of pro- 
jection lenses, in 
the form of a 
paper read be- 
fore the Soci- 
ety of Motion 
Picture Engi- 
neers at its 
meeting in Day- 
ton, Ohio, Octo- 
b e r 12, 1920. 
This paper ap- 
pears in the 
Dayton trans- 
actions of the 

"Losses of 
light in projec- 
tion lenses may 
occur for two 
reasons : 

"First, geometrical arrangement of power, distances, etc., 
sizes of lenses ; 

"Second, physical reasons, material the lenses are made of, 
conditions of glass surfaces, etc. 

"It is the physical conditions with which we are con- 
cerned now. 

"When light strikes a border surface between two opti- 
cally different media, water and air, for instance, a part of 
the light enters the water and illuminates the bottom or the 
containing vessel, while another part is reflected back into 


93* Q 
95% ^ 



99% > 





o // /e /a u is /s ir /a 

Figure 35. 



the air. So is part of the sunlight which strikes a window 
passed into the room, while another part is reflected upon 
the street. The ratio of reflected to transmitted light de- 
pends upon the refractive indices of the media, as well as 
upon the finish of the surface. 

"The greater the difference in refractive index, the more 
light is reflected and the less light is transmitted. If we 

have two horizontal 
surfaces, side by side, 
one formed by water, 
the other of glass, 
and observe the 
amount of sunlight 
reflected, we will 
find the glass surface 
to be a more efficient 
reflector than the 
water surface, and at 
the same time the il- 
lumination inside the 
water will be greater 
than inside the glass. 
"The amount of re- 
flected light also 
changes with the 
angle of incidence. 
The more nearly per- 
pendicular the light 
source is to a sur- 
face, the less the 
amount of light reflected. From this it is evident that the 
conditions for passing the greatest amount of light become 
most favorable when the refractive index is as low and the 
angle of incidence as small as possible. Remembering that 
the angle of incidence is the angle between the direction of 
the ray of light and the perpendicular at the point of incident, 
we see that the nearer the ray of light is perpendicular at the 
point of incidence the smaller is the angle of incidence. 

"The action of a lens depends on the refractive index of 
the material it is made of. To get any lens action at all, 
the refractive index has to be greater than that of air, the 
value of which is set at 1.00. We therefore shall always have 
a certain loss of light when it passes through a lens sur- 
face. The amount of this loss is determined by means of a 

Figure 36. 

Shows the curves for the loss of light in a 
double convex lens of refractive index 1.51 at 
and between the surfaces. 



formula which was derived by Fresnel, the famous French 
mathematician and optician. He found for perpendicular 
incidence the following equa- 
tion : 


I — I 1 = 2 


Wherein I is the amount of 
light falling upon the surface ; 
I 1 the amount of light trans- 
mitted by the surface; and n 
the refractive index of the 
medium. The difference I — 
I 1 is the loss of light at the 
refracting surface. The curve 
in Fig. 35 shows, for refrac- 
tive indices varying from 1.0 
to 1.8, the intensities of the 
reflected and transmitted light. 
"The amount of variation in 
loss of light with variation 
in the angle of incidence is 
set forth in the following 
table, in which it is assumed 
that the light is unpolarized, 

an assumption which satisfactorily represents practical con- 
ditions : 

Figure 36A. 

Shows the curves for the loss of 
light in a double convex lens of 
refractive index 1.61, at and be- 
tween the surfaces. 

Table 6A 

Angle of 


Per Cent. 

Per Cent. 


Crown 1.51 

Flint 1.61 

Loss 4.13 

Loss 5.46 






" 10.69 


" 17.29 

" 18.97 


" 38.92 

" 40.30 


" 100.00 

" 100.00 

Caution Note.— "Crown 1.51" and "Flint 1.61" refer to re- 
fractive index of those glasses. 

"The refractive indices of the glasses used in projection 
lenses, which are crown and flint, vary from about 1.51 
to 1.61. 

"From table 6A we learn that the loss of light per surface 
for perpendicular incidence is 4.13 per cent, for crown glass 



the refractive indice of which is 1.51. Whereas for flint glass 
the refractive indice of which is 1.61 the loss amounts to 
5.46 per cent. 

"The angle under which light rays strike the surfaces of 
a projection lens varies from 0° to about 30°, and fortunately 
for the simplicity of this discussion the variation of the 

loss within these 
angles is negligi- 
ble, as may be 
seen in the table. 
We may there- 
fore say that the 
loss of light per 
surface on a 
crown glass lens 
amounts to 4.2 
per cent., and on 
a flint glass lens 
it is 5.5 per cent. 
"The refractive 
index of Cana- 
dian Balsam is 
a pproximately 
equal to that of 
crown glass, 
therefore if a 
crown lens of 
1.51 and a flint 
lens of 1.61 be 
Figure 36B. cemented to- 

Shows the curve for the loss of light in a cemented gether, the loss 
doublet consisting of a lens with refractive index . -~fl «„*:«*, o<- 
1.51 and another lens with a refractive index 1.61 D y reflection at 
at and between the surfaces. a cemented sur- 

face is very 
much smaller than at surfaces bordering on air. The refractive 
index of Canadian Balsam being practically the same as 
thjit of crown glass, no loss is incurred in the passage from 
the crown glass lens into the layer of balsam, while between 
the balsam and the flint glass the difference amounts to only 
0.1 per cent., which means the total loss is almost negligible. 
"For a number of glass surfaces in air, each having about 
the same loss by reflection, the total loss may be arrived at 
by using the compound interest formula. If the loss, p, for 



one surface is represented as a fraction of the initial in- 
tensity, the loss for k surfaces will be 1 — (1 — p)k. 

"The loss by absorption depends on the nature of the 
glass and on the thickness of the lenses. Good crown glass 
will not absorb more than 0.5 to 1 per cent, per centimeter 
thickness, while flint glass of 1.61 refractive index will run 
a little higher, from 1 to 1.5 per cent, per centimeter. 

'The lighter varieties of flint glass, of indices between 
1.54 and 1.61, have absorptions between these limits, while 


""""" 1 1 1 1 1 LI 1 1 1 1 1 1 


Figure 36C 

the absorption of poor glasses runs as high as 3 and 4 per 
cent, per centimeter. 

"Considering a crown glass lens of an average of 5 mm. 
thickness, the losses by reflection and absorption total up 
as follows: At first surface 4.2 per cent., by absorption .5 
per cent., at last surface 4 per cent., or a total of about 9 
per cent, per lens. 

"With a flint lens of 1.61 refractive index we obtain the 
following data : Loss at first surface 5.5 per cent., by absorp- 
tion .5 per cent., at last surface 5.2 per cent., or a total of 
about 11 per cent, per lens. 

"For a cemented lens consisting of cemented crown and 
flint elements we find 4.2 per cent., .5 per cent., .1 per cent., 
1 per cent., 5.2 per cent., or again, a total of about 11 per 
cent. Figures 36, 36A and 36B represent these conditions 



"In this way it is easy to form an opinion as to the losses 
by reflection and absorption in any combination. In gen- 
eral it is safe in forming an estimate to assume the total 
per single lens, or per combination of cemented lenses, as 
they occur in projection lenses, at about 10 per cent. 

"If the workmanship and material are approximately the 
same, and no difference exists in the geometrical arrange- 
ment, diaphragming, etc., of the lenses, the amount of light 
passed through is very nearly the same in all lenses that 
have the same number of reflecting surfaces. 

RESULT OF DIRTY LENSES.— 'A very important factor 
in the performance of a lens is the condition of its surfaces. 
Scummy and dust-covered lenses give gray pictures, with no 
contrast, exactly in the same way that photo lenses will not 
take good pictures if dispersed light from finger-marks on 

Figure 36D. 

the lens is allowed to reach the plates. It is therefore of 
the utmost importance that in comparative tests the lenses 
be perfectly clean. 

"I may mention briefly the fact that there exists a con- 
dition of lens surfaces which is helpful in the way of re- 
ducing the amount of reflected light and increasing the 
amount of transmitted light. Such surfaces have an 
iridescent appearance caused by chemical action. It had 
been discovered accidentally in process studios that such 
lenses allow shorter exposures. Upon investigation it was 
found that the amount of transmitted light actually had been 
increased. Unfortunately such action takes place for the 
most part on glasses which have comparatively great absorp- 
tion co-efficients, and not generally used for projection 
lenses. Any thought of reducing losses in projection lenses 



by these means will probably have to be dismissed, although 
the amount of loss by such treatment can be reduced by as 
much as 50 per cent. 

"In conclusion I may say that all the statements made 
above are borne out by practical tests, not only in compara- 
tively simple combinations like projection lenses, but also 




Figure 36E. 

in the most complicated apparatus like range finders, sub- 
marine periscopes, etc." 

(Note — We have made a few changes in Dr. Kellner's 
language to lessen his technicality for our readers, but his 
meaning has in no degree been altered. — Author.) 

will be explained in later pages, the diameter of the pro- 
jection lens is a matter of large importance. Under condi- 
tions where the amperage is high and the working distance 
of the projection lens long a small diameter objective means 
heavy loss in efficiency — waste of light — as well as uneven 










Figure 36EE. 



illumination of the screen, with consequent loss in "depth" 
in the picture itself. On the other hand, it may be accepted 
as fact that the small diameter objective makes for improve- 
ment of the picture, gives much greater depth of focus— 
an important matter indeed where there is considerable 
angle to the projection — and in general sets up a better con- 
dition with regard to the revolving shutter. It follows that 

/?S?£rt3L r 





Figure 36F. 

any diameter greater than that necessary to accommodate 
the actual picture bearing light beam is not only undesirable^ 
but distinctly bad. 

We might, however, here repeat that it is impractical to 
make projection lenses of greater free diameter than half 
the focal length (E. F.) of the lens. This is a limitation 
which often causes a bad condition, especially in the shorter 
focal length lenses because, except in the case of very short 
E. F. lenses, only two diameters are made in practice. 
Whether it is practical to make all projection lenses up to 
5-inch with a free diameter equal to half the E. F. of the 
lens or not, we do not know. However, if it can be done 
without sacrificing any valuable quality of the lens it cer- 
tainly should be done. 

DISTORTION.— Projectionists should carefully test their 
projection lenses for distortion. This may readily be done. 
First secure a perfectly flat piece of mica, commonly known 
as isinglass, three or four inches in length. Trim it to the 
exact width of a film and then, using a coarse needle, 
scratch straight lines on the surface of the mica, checker- 
board fashion, as per Fig. 36C. 



Having done this, place the mica over the projector aper- 
ture and close the film gate, being careful that the mica 
lies perfectly flat over the aperture, just as the film would 
lie. Next, raise the fire shutter and project the lines on the 
surface of the mica to the screen, and with a tightly 
stretched cord, test the lines on the screen to see that they 
are perfectly straight. At A, Fig. 36C, the lines are straight, 
hence there is no distortion. At B there is what is known as 
"barrel distortion/' which amounts to a curvature of the 
lines. The lens which projects B is not a good lens, whereas 
the lens which projects A is perfect, insofar as has to do 
with distortion. 

CAUTION. — Lines on the mica need not be perfectly square 
with each other, but each line must be perfectly straight. 

FOCUSING. — The objective must focus all rays of light 
emanating from a given point of the film to a corresponding, 
though magnified, point on the screen. 

As has already been set forth, the film represents one 
conjugate foci point of the objective and the screen the 
other. As you all know, if the distance of the objective from 
the film be in the slightest degree altered by means of the 
focusing screw, the definition of the picture on the screen 
is altered. The reason for this is diagrammatically repre- 
sented in Fig. 36D. 

#SS£MBL y 


/SALS 5/Z£ 

Figure 36FF. 

Here A is the film, B the objective, and 2 the screen posi- 
tion, with the picture out of focus because of the fact that 
lens B is too far from film A. The rays emanating from 
points in film A are thus focused at 1 instead of at 2. Be- 
tween B and 1 the rays converge until they finally meet at 



1, but having met at 1 they cross and continue in straight 
lines, diverging, so that at position 2 each pin point of the 
film is represented by a bundle of rays, or a circular spot of 
light. Now, if we move lens B just a little closer to film A, 
the point of focus will then be at 2, instead of at 1. A 
study of this diagram should inform the projectionist as to 



Figure 36FFF. 

exactly what takes places when he focuses the picture on 
the screen. 

"DOCTORING" LENSES.— The author has often been 
asked this question : "Can the E. F. of a lens be altered by 
shortening or lengthening its barrel in such a way that the 
distance between the back and front factor will be changed"? 

This question must be answered both yes and no. The thing 
may be done, but only at the expense of ruining the lens, 
insofar as concerns its ability to do high class work. 

is often asked, "Can a given projection lens be used to pro- 
ject a picture at different distances"? The answer is, yes. 

But the same objective will not project the same size pic- 
ture at different distances. 

If the distance be made less, the picture will be smaller, 
and vice versa. Changing the distance of projection will 
also alter the working distance of the lens — distance from 
back factor to film. The shorter the distance between the 
lens and the screen the further the lens must be from the 
film, and vice versa. 

The light beam diverges on its way from the projection 
lens to the screen. It is easy to figure how much change in 


size of picture will be accomplished by moving the screen 
a given distance nearer to or further away from given fens, 
since the divergence of the ray emanating from any given 
projection lens is a fixed quantity, and does not alter. See 
"Same Lens, Different Distance," page 154. 

MEASURING LENSES.— It is not only desirable, but 
essential that the projectionist understand how to determine 
the focal length of the various lenses he is called upon to 
handle, with at least a reasonable degree of accuracy. 

Condenser lenses, being uncorrected and containing con- 
siderable spherical aberration, cannot be measured with any 
great degree of accuracy by following method but the 
focal length of plano-convex condenser lenses may never- 
theless be obtained with sufficient accuracy for practical 
purposes. Select a room with only one window, or else 
darken all the windows in a room but one, leaving this one 
open. On the wall opposite this window pin a sheet of 
white writing paper. Hold the lens to be measured in front 
of the paper screen thus established, with its flat side 
toward the screen, and carefully focus some distant object, 
such as a building or tree, on the paper screen. Be careful 
to hold the lens as nearly as possible square with the screen. 
Having focused the object selected as sharply as possible, 
measure the exact distance from the flat side of the lens to 
the screen, after which reverse the lens so that its curved 
side is toward the screen, focus the same object and again 
measure the distance from the flat side of the lens to the 
screen. Add these two measurements together and divide 
their sum by two. 

This result wiJl be near enough to focal length of the lens 
for practical purposes. 

CAUTIONw — In the foregoing it is essential to accuracy 
that the object focused be not less than 100 feet away, be- 
cause of the fact that the focal length of the lens is the 
distance from its optical center at which it will focus parallel 
rays of light, and with less than 100 feet the rays entering 
the lens from a point on any object cannot be said to be 

It is, however, possible to secure fairly accurate results by 
focusing an object which is only 50 feet away, and no great 
error will result even if the object fixed be only 15 or 20 
feet away. But always focus an object 100 or more feet 
away if it is possible to do so. 

For example, suppose the measurement with the flat side 


of the lens to the screen to be 6 inches, and the second 
measurement, with the curved surface toward the screen, 7 
inches. Then, 6+7=13, and 13^-2=6^, hence the focal length 
of the lens is 6y 2 inches. 

A bi-convex condenser lens may be measured the same as 
the plano-convex, except that a single focusing is sufficient. 
The measurement should be from the center of the lens to 
the screen. 

The Meniscus bi-convex cannot be measured in the same 
manner as a plano-convex, because its optical center lies 
outside the lens. 

The motion picture or stereopticon projection lens has 
these two distinct measurements: (1) the "working distance," 
which is the distance from the object (film or slide) to the 
first surface of the first factor of the lens when the image on 
the screen is in sharp focus, and (2) the "equivalent focus" 
(E. F.), which is one-fourth the distance from an object to 
its image when the object is focused by the lens being 
measured and the image and object are of the same size. 

The working distance of the motion picture projection lens 
is of great importance to the projectionist, in that it has 
directly to do with necessary lens diameter, as will be set 
forth in figure 47, page 182. 

The equivalent focus of the motion picture projection 
lens is the factor used in ordering lenses to project a given 
size picture at a given distance. 

working distance of a projection lens, place it in the pro- 
jector, thread in a film and focus the picture sharply on the 
screen. Remove the film, and running a rule through the 
projector aperture, measure the distance from the aperture 
plate (film plane) to the first surface of the projection lens. 
This measurement will be the "working distance" of the 
lens, wrongly referred to as the "back focus." See "Back 
Focus," page 21. 

focus (E. F.) of a projection lens may be measured accurate- 
ly as follows: Remove the projector mechanism and in the 
position occupied normally by the projector aperture, 
mount a sheet of metal in which an opening about .75 of an 
inch square has been cut. Next support the lens to be 
measured at a distance about twice its supposed E. F. on 
what would be the screen side of the aforesaid opening. 
Then, with the light projected through the opening in the 


usual way, hold a small screen, preferably of a dull black 
or very dark non-gloss color on the screen side of the lens. 
Move lens and screen until the image of the opening on the 
screen is precisely the same size as the opening itself, where- 
upon one-fourth the distance from aperture to screen is the 
exact E. F. of the lens. 

To obtain this measurement properly what is known as 
an "optical bench" is necessary. We do not submit it as 
having much actual value to the projectionist, but merely 
for the sake of completeness. 

TO MEASURE ANY LENS.— The following method of lens 
measurement was supplied by John Griffith. It may be used 
to measure the focal length of any simple lens, such as a 
plano-convex, bi-convex or meniscus, or it may be used to 
measure the E. F. of compound lenses, such as projection 

To apply it, two things are necessary. The EXACT dis- 
tance lens to screen, and a stereo slide on which is an 
opaque mark precisely .5 of an inch in length. 

As to the slide, it may well form a permanent part of the 
projectionist's tool outfit. It may be made in several ways, 
the easiest of which is to cut a piece of thin metal precisely 
.5 of an inch in length, binding the same up between two 
cover glasses. 

Another way is to make a scratch mark on a cover glass. 
A third way is to cut a slit in a metal slide, the latter being, 
of course, harder to make, but once it is finished it is in the 
nature of a permanent tool. However you may choose to 
make your test slide, be certain the mark is exactly one- 
half inch long, running, of course, horizontal — lengthwise on 
the slide. 

Next it is necessary to measure the exact distance from 
the lens to that part of the screen to which the line will be 
projected, the latter because of the fact that under some 
conditions there is a decided difference in distance from lens 
to different parts of the screen, also a difference in length 
of the projected line. This measurement must be reduced 
to inches. We would suggest that the distance from screen 
to inside wall of projection room at a point midway of the 
vertical height of the lens port be made a permanent part of 
the projection room records. It will then be a simple mat- 
ter to add to that measurement the distance from that point 
to the lens. 

Having the distance measurement and slide, project the 


latter to the screen with the lens to be measured (a con- 
denser lens may be used for a projection lens, hence you can 
measure condenser lenses by this method) and measure the 
exact length of the projected line on the screen. The rule is: 

As many times as .5 is contained times into the length of 
the projected line, measured in inches, the focal length of 
the lens, if it be a simple lens, or the £. F. of the lens, if it 
be a compound lens, such as a projection lens, will be con- 
tained into the distance of projection (lens to screen) meas- 
ured in inches. 

For example: The projected line, or mark, is found to 
measure 7 feet S.3 inches, or 92.3 inches. The distance from 
the lens to the screen (distance of projection) is 100 feet, 
'or 1,200 inches. We then have 92.3-^.5=184.6 (dividing by 
.5, or y 2 , is exactly the same as multiplying by 2) and 
1200-^-184.6=6,5 plus a small fraction. Therefore our lens is 
a trifle over 6.5 inch focal length, if a condenser lens, and a 
bit more than 6.5 E. F., if a projection lens. 

of picture any projection lens will project at a given distance 
is dependent upon the E. F. of the lens. The shorter the E. F. 
■of the lens the wider the picture it will give at a given dis- 
tance of projection, thus : A 4 inch E. F. projection lens will 
project a much wider picture at sixty feet than will a 6 inch 
E. F. lens. 

jection lens may be used to project at different distances, but 
the resultant picture will vary in size directly as the distance 
of projection. If the size of the picture a lens projects at 
a given distance is known, the size it will project at any 
other distance may be computed, thus : 

Suppose a lens projects a sixteen foot picture at seventy 
feet. Reducing the size (width is always meant when pic- 
ture "size" is named with only one dimension given) to inches, 
we find the picture to be 16x12=192 inches wide. Since 
the distance to the lens is 70 feet, it follows that the ray 
from the lens spreads 192-^-70=2.742857 inches per foot. We 
have then only to multiply any desired distance of projection 
by 2.742857 to know precisely what size picture that lens 
will give at that distance. 


SIZE.— To compute the required E. F. of lens to project a pic- 


ture of given size (width) at a given distance, proceed as 
follows : 

Measure the width of aperture of your projector accurate 
ly (if it is a stereopticon lens, then the standard slide mat 
width, 3 inches, is used) by means of an inside caliper, though 
if the projector be of late model we may take the aperture 
width at .90625 (29/32) of an inch with assurance of pretty 
close accuracy. Next measure the exact distance from the 
center of the lens barrel to the screen. Multiply the distance 
from the lens to the screen, in feet, by the width of the aper- 
ture, in fractions of an inch, and divide the result by the width 
of the picture you desire, in feet. 

The result will be the E. F. of the lens required to project 
a picture that width. It will be as close to it as you or any 
one else can get by figuring. If the lens itself is accurate 
as to E. F., the result also will be accurate. 

For instance : Suppose we desire a fifteen foot picture at 
sixty feet. The projector aperture is found to be .90625 
(29/32) of an inch wide — the new standard. We first multiply 
the distance from the screen, in feet, by the width of the 
aperture, .90625, which gives us 54.3750. Dividing this by 
the width of the desired picture in feet we get 3.625. We 
would therefore require a lens of 3.625 inches E. F. to pro- 
ject a picture exactly 15 feet wide at exactly 60 feet. 

It would probably be impossible to find a lens marked 
exactly this focal length. The most practical method is to 
determine the width of the picture you want and the exact 
distance from the lens to the screen, supplying this data to 
the lens dealer. He will probably be able to select a lens 
from his stock which will meet your requirement. 

The stereopticon lens is figured in exactly the same way, 
except that instead of measuring the aperture width we 
accept 3 inches as the standard slide mat width — the slide 
mat being, in this case, the aperture. 

For the benefit of our readers we append the formula by 
means of which certain other factors may be figured : 

The size of the image which a lens of given E. F. will 
project at a given distance may be found by multiplying the 
difference between the distance lens to screen and the focal 
length of the projection lens (E. F.) by the width of the 
aperture, and dividing the product by the E F. of the lens. 
For example, let L equal the projection distance, 40 feet 
(480 inches); S the slide mat (3 inches), and F the E. F. 
of the lens, which we will assume to be 12 inches. We then 


S (L-F) 

have the formula d = in which d is the width of 

the image in feet. Substituting for the letters their known 
3 (480—12) 

value we have: d = = 117 inches or 9.75 feet; 

that being the width of a picture a 12-inch E. F. stereopticon 
lens will project at 40 feet, provided always that the slide 
mat be exactly 3 inches wide. If, however, the mat be more 
or less then 3 inches wide, then the picture will be more or 
less than 9.75 feet wide. The image size a motion picture 
projection lens would give at a given distance may be fig- 
ured in the same way, using the exact width of the projec- 
tor aperture instead of the slide mat width. 

If we know the size of the image, the width of the aperture 
and the E. F. of the projection lens, we may figure the dis- 
tance to the screen as follows : 

F (d+S) 

Suppose, for instance, we have a 12-inch E. F. stereopticon 
projection lens, and want a picture 117 inches wide. Sub- 
stituting figures for the above formula we have 
12 (117+3) 

L = = 480 inches or 40 feet. 

We would therefore be obliged to place the screen 40 feet 
from the lens in order to get out the 117-inch picture desired. 

LENS QUALITY.— In the past the author has advised the 
installation of high grade projection lenses, meaning by "high 
grade" an expensive, highly corrected lens. We still believe 
firmly in high quality lenses, but are convinced that some of 
the manufacturers of lenses in the United States are now 
producing projection lenses quite good enough for all prac- 
tical purposes, insofar as quality be concerned. The mount- 
ings of these lenses are excellent and the resultant image 
is very good indeed. With regard to anastigmat projection 
lenses, we believe they are only desirable in comparatively 
short focal length lenses. An anastigmat projection lens of 
long or even moderately long focal length does not seem to 
present any particular advantage. 

All projection lens manufacturers of prominence were in- 
vited to submit such data as would be of benefit to the users 



of this book. The Koll-Morgan Optical Company, Brooklyn, 
New York, submitted the excellent data found in Figs. 
36E to 36FFF. From these excellent drawings all necessary 
data may be had. 

Under "Assembly" the lenses are shown mounted and un- 
der "Optics" we see the lenses of each lens unmounted, to- 
together with the dimensions in millimeters. It will be 
observed that the Snaplite lenses have five diameters, and 
that they range from 2.5 to 11-inch E. F. They are very 
well mounted and by comparison with some other makes of 
projection lenses are very highly corrected. They have the 
approval and indorsement of the Projection Department of 
Moving Picture World and of the author of this book. 

Bausch & Lomb 
, manufactures pro- 


Optical Company, Rochester, New York 
jection lenses. 

The company SERIES I 
supplied very 
complete data 
same. The 
Bausch & 
Lomb lenses we 
have found to 
be highly cor- 
r e c t e d, well 
made and well 
mounted. They 
are cordially 
commended for 
all conditions to 
which their 
diameters are 
suited. The il- 
lustrations show 
clearly the as- 
semblage of 
lenses and their 
data gives all 
free diameter. 

It will be noticed that in series O and I lenses of the 
same E. F. may be had in two different diameters. We 
could hardly imagine the condition where the O d : ameter 


& L. 

Lenses — Exactly 

relation to each other, 
necessary information as 

L- ot/r9/MrJf/Affrm— «| 


one-half size of actual 

while the tabulated 
to both outside and 



would be better, unless it be in the case of a heavy angle 
projection which compelled great depth of focus. 

Figure 36GG. 

B. and L. lenses, series 11, exactly one-half size of actual lens. 

lens and projector manufacturers put out lens tables pur* 
porting to give the size of image a lens of given E. F. wHl 
project at a given distance. We did not incorporate these 
tables in this work because we do not regard them as having 
any practical value. In the case of the stereopticon, slide 
mat widths vary so much that the results from such tables 
are little more than guesswork. In the case of the motion 
picture the average exhibitor or projectionist wants a picture 
of exactly the size he has decided upon, at an exactly pre- 
determined distance, and for such accuracy he cannot rely 



upon tables. Hence the tables are not, we feel, a desirable 

Note : To change mm. measurements to inches multiply 
same by .03937. 












E.F. mm. 

mm. No. 





4" 38.0 

36.0 10 





4y 4 " 










































6" " 













Front Combination 

Back Combination 


E.F. Diameter. Diameter 



Outside Inside 








































7V 2 " 












8y 2 " 











Dimensions of Bausch & Lomb Motion Picture Lenses, compiled f<W 
F. H. Richardson. 


From time to time, some inquiry is had as to the practicability 
of projection lenses of variable focus — lenses of which the E.F. 
may be altered by changing the distance between the front 
and rear elements. 

Such a lens can be and is made, but unless both its factors are 
so accurately corrected and mounted that its cost is increased 
to three to four times the cost of an ordinary fixed focus pro- 
jection lens of highest quality, the lens cannot compete with the 
fixed focus projection lens of good quality, either in definition 
or power of illumination. 

We do not adviaC the use of variable focus projection lenses, 
except in cases where the user is willing to sacrifice quality 
and power of illumination to secure the adjustable feature. 

WARNING: Such lenses are and can only be adjustable over 
a very narrow range — probably not to exceed one inch focal 


Practical Projection Optics 

IMPORTANT: The following has principally to do with 
the plano-convex condenser, though in some points it will apply 
more or less equally to the Cinephor parabolic condenser, and 
the reflector type of arc lamp light source, both of which will 
be dealt with separately. 

THE optical system or optical train of the motion picture 
projector is made up of two entirely separate elements, 
or lens systems, which are joined together in such a 
way that one system (the projection lens) receives at one of 
its conjugate foci points the magnified and more or less out 
of focus image of the light source projected by the other 
system, the condenser. 

The optical train of the projector consists of the con- 
denser, (A) the office of which is to pick up as great a num- 
ber as possible of the diverging rays from the light source, 
refract them into converging rays and concentrate them into 
what is known as the "spot" at the aperture of the pro- 
jector, and (B) the projection lens, the office of which is to 
pick up the film image illuminated by the concentrated rays 
from the condenser and project it to a focused image upon 
the screen. 

Joining these two optical systems is easy if we disregard 
the item efficiency, but to join them in such way that they will 
work together with the greatest possible degree of efficiency 
is somewhat intricate, and in some conditions met with, an 
almost impossible problem. Yet, unless the two lens systems 
are made to work together with the least possible loss, the 
waste in light, which means waste in electric power, may be a 
very serious matter, indeed. 

We do not believe the optical system of the modern pro- 
jector can be judged by ordinary optical standards, because 
of the fact that the conditions under which the lenses must 
work are themselves, in almost every particular, highly 
abnormal. In the case of the condenser the heat is excessive 
under any condition, and when the amperage mounts to 80, 
90, 100, and even in some extreme cases higher, a condition 
is set up which calls for special treatment. In the case of 
the projection lens, it may, under some conditions, receive 
along with the film image it is to project to the screen, a 
more or less in focus image of the floor of the electric crater, 
or Mazda filament projected by the condenser system, and 
unless the projectionist very thoroughly understands his 


business it is more than likely also to receive only a portion 
of the light passing through the aperture, which means an 
unevenly illuminated image on the screen, and in addition to 
all this it will receive more or less stray light reflected from 
the edge of the aperture. Nor is this all the story of the 
difficulties of the motion picture projection lens, because due 
to manufacturing limitations, and the further fact that it 
must work in conjunction with the revolving shutter, its 
diameter is necessarily limited. Its diameter is also, to an 
extent, limited by the further fact that too-large diameter 
makes for lack of depth of focus in the screen image, which 
latter is a very important item indeed, if the projection lens 
be considerably above, or to one side of the center of the 

Jn the following we shall do our best to convey a clear 
understanding of practical projection optics, which is, we can 
assure you, no easy task. 

The collector lens element of the condenser (collector lens 
is the one next the arc) must pick up the light emanating 
from the light source, which approaches the lens in diverging 
rays, see Fig. 36H. 

THE LAW OF LIGHT.— Light diminishes in intensity as 
we recede from the source of light. If the luminous source 
be a point, then the intensity diminishes as the square of 
the distance increases. If we call the quantity or amount of 
light received by a certain given area at the distance of one 
foot 12 C. P., then at a distance of 2 feet its intensity would 
be J4th of 12, or 3 C. P., and at a distance of 3 feet it would 
be l/9th of 12, and at a distance of 10 feet it would be l/100th 
of 12, and so on. This is the true meaning of the law, which 
reads : 

With an open light source, light intensity varies inversely 
as the square of the distance from its source. 

This law has its base in the fact that light is propagated, 
or travels in straight lines, and naturally those lines are 
diverging — separate from each other as they go forward. 
You may prove the law as follows : Place a light source, 
which may be an incandescent globe, though the law holds 
strictly true only with point light sources, at a distance of 
nine feet from a white wall. Hold a cardboard six inches 
square at a distance of exactly 2.25 feet from the light source, 
which is one-fourth the distance from light to screen, and 
you will find the area of the shadow of the cardboard which 
is cast upon the wall to be sixteen times that of the card* 



board itself. It therefore follows that the amount of light 
falling upon one square inch of the cardboard would be 
sixteen times as much as would be the light falling upon a 
similar area of a screen located four times as far away from 
the light source. The operation of this law and the practical 
effect of distance of crater from face of collector lens is 

very clearly 
shown in Fig. 
36H, in which F 
is an ordinary 
Ay 2 inch diam- 
eter plano-con- 
v e x collector 
lens, having a 
free opening of 
4^4 inches, its 
face located 2y 2 
inches from the 
center of the 
crater floor. 

With the cra- 
ter thus estab- 
lished, it re- 
quires no unus- 
ual power of 
discernment to 
understand that 
the lens will re- 
ceive all light 
rays within the 
That much may be very 

Figure 36H. 

space bounded , by lines A — B. 
readily understood. 

Remembering that once the light rays have left the source 
(crater floor) they travel in absolutely straight lines to in- 
finity, it is readily seen that if we move lens F back to the 
position occupied by lens C it cannot and will not receive as 
great an amount of light as it did in the first position. In fact 
we believe that even the most obtuse will not dispute the prop- 
osition that a lens in position C, Fig. 36H, must of necessity 
have the diameter shown (7*4 inches) in order to receive 
as great an amount of light as is received by the 4^4 inch 
lens opening in position F. 

The perpendicular dotted lines represent the faces of lenses 
at different distances from the center of the crater as shown, 


and the diameter each lens would have to be in order to have 
light collecting power equal to the 4% inch lens in position 
F is also given. It will be observed that at 3 inches a 5 inch 
diameter lens would be required, and at 3 l / 2 and 4 inches, a 
5% and 6 l / 2 inch lens would be required. We believe this 
diagram will be rather startling to some projectionists who 
have paid no especial attention to the distance of their crater 
from the lens, and who have given no study to the practical 
operation of the law we have so often quoted in the Pro- 
jection Department of Moving Picture World, which is again 
quoted at the beginning of this subject. 

With the foregoing in view the importance of getting the 
light source as near the collector lens as possible is clearly 
seen, since, due to reasons which will be hereinafter ex- 
plained, the diameter of the condenser lens is limited. 

CAUTION. — In the foregoing connection let it be clearly 
understood, we do not mean that if the arc be retarded a 
given distance further away from a collecting lens of a given 
diameter the light loss would be as great as might be in- 
ferred from an examination of diagram, Fig. 36H. Fig. 36H 
is absolutely correct, but the fact remains that about 63 per 
cent, of the light is concentrated upon 50 per cent, of the 
area of the center of the collector lens, hence we could 
retard the arc a sufficient distance to equal a loss of 50 per 
cent, of the area of the collector lens and yet only lose about 
37 per cent, of the light. 

To enter into a detailed explanation of the reasons of this 
would not, we believe, be wise because the matter is largely 
one of geometry and would be very difficult for the mind of 
the average man to grasp. However, let it be clearly under- 
stood that the concentration of 63 per cent, of the light into 
50 per cent, of the area of the collector lens in no wise 
impairs the correctness of the proposition set forth in Fig. 
36H. It does, however, operate to modify the amount of loss 
when we deal with a collector lens of given diameter at 
different distances from the light source. 

The first task in the adjustment of the optical train of the 
projector is, therefore, to ascertain how near an electric arc 
may be located to the collector lens without causing the lens 
to break under the influence of rapid expansion and con- 
traction caused by excessive heat. It is this factor which 
forms tne basis of the lens charts, and the arc is auto- 
matically placed the minimum distance it is possible to 
establish the crater of an electric arc of a given amperage 


from a lens when lens charts are used. It is partly because 
of the fact that when Moving Picture World lens charts are 
applied this minimum distance is automatically established 
that we earnestly recommend all projectionists whose amper- 
age is within the limits of the charts to use them. It is 
hardly a practical thing for the projectionist to determine 
by experiment exactly what the minimum distance is, and 
then figure out the lens combination necessary to give the 
proper size spot at the proper distance. 

OPTICS OF THE CONDENSER.— The condenser may be 
composed of either two or three lenses, but the universal 
practice both in the United States of America and Canada is 
to use only two, though to some extent in England the three- 
lens combination is used, and in Germany we understand 
that it is used almost exclusively. In the two-lens com- 
bination the lens next the light source is known as the "col- 
lector" lens, and the front lens is known as the "converging" 
lens. Where there is a three-lens condenser the lens located 
between the collector and converging lenses is known as 
the "centre" lens. There are great possibilities for light loss 
inherent in the condenser itself. 

DISCOLORED LENSES.— When light passes through per- 
fectly clear, colorless glass of good quality, only a very 
small percentage of its energy is absorbed. In this we refer 
only to the actual passage of light through the glass — not the 
loss incident to reflection as the light enters and leaves the 
lens. Competent authorities place this absorption loss at 
about 1 per cent, per each inch of glass, for high grade 
glass. Other competent authorities place it at about .5 of 
1 per cent, per centimeter, which is a little higher than 1 
per cent, per inch. 

When a lens is discolored experiments have proven that 
the actual absorption loss is not greatly increased. The fol- 
lowing figures are from measurements of discolored con- 
denser lenses and give some idea of what it amounts to : 

Kind of DiscolorationFocal Length Percentage of light passed by 

Piano- Convex discolored lens as compared to 

Lens. light strength without any lens 

at all. 

Slight green 

7 7/16 inch 

88.8 per cent. 

Pressed lens, yellow 

7 1/16 " 

89.2 " 

Pressed lens, yellow 

7 1/16 " 

88.8 " 

Dark green 

6 11/16 " 

87.2 " 

Many other lenses were measured, but the results were all 
within a range of 89.2 high and 87.2 low. When it is con- 


sidered that both reflection and ordinary absorption losses 
are recorded together with the color loss, it is seen that the 
color loss must be slight. 

It does not, however, at all follow that color in condenser 
lenses is not harmful. As a matter of fact it is harmful, 
because it changes and injures the color value of the light. 
True, the color may not actually absorb very much of the 
total light, but it does alter and lower its value for pro- 
jection purposes. For instance: The light from a condenser 
having a greenish cast is "muddy." It is not clear and bril- 
liant, hence its value for projection purposes is lowered. 

It seems, however, that the pink color sometimes found in 
condenser lenses which have been used for a time may pos- 
sibly not be detrimental, but in fact beneficial. Now that we 
know color in the lens does not, as formerly presumed, 
necessarily mean excessive loss of light, it might be well to 
experiment with color to some extent, but if this is done 
the colors must be confined strictly to those tending to 
mellow the light, without seriously affecting its brilliancy. 
A slight pink tinge might be beneficial. At least this dis- 
covery opens up a field for investigation, though the fact 
that tinted film must be projected cannot be overlooked. 

AVOID COLOR. — But in any event, except for experi- 
mental purposes, until this matter be finally settled, the pro- 
jectionist should avoid all color. In purchasing lenses, any 
lens which shows the slightest trace of color, when looked 
through EDGEWISE, should be promptly rejected. 

In examining lenses for discoloration, however, be very 
certain that any apparent color is actually in the lens itself, 
and not due to surroundings. 

THROW THEM AWAY.— The projectionist should always 
look through his condenser lenses edgewise when cleaning 
them, and should any discoloration appear he should promptly 
discard the lens, explaining to the manager that the lens 
may in a very short time use up the price of a new lens in 
light brilliancy. This should invariably apply to green, and 
should be the rule for all colors, until such time as it may 
become an established fact that certain colors are desirable — 
if it ever is. 

PITTED LENSES.— When using certain types of carbon 
there is a decided tendency to pitting of the collector lens. 
The question is often asked, does this pitting cause light loss? 
The answer is, to a certain extent, yes, but this loss is not 
as large as might be supposed. The pit consists of a spot 


from which the polish of the lens has been burned by impact 
with incandescent bits of carbon or metal. The light rays 
striking the lens at this spot are diffused, and a greater per- 
centage of the light is reflected from such a spot than would 
be reflected if the polish were perfect. All the light not re- 
flected by the unpolished spot of course passes through the 
lens, but reaches its other surface in the form of more or less 
perfectly diffused light and it is problematical just what per- 
centage of it the three remaining lens surfaces which it must 
pass through will redirect and converge to the spot. 

In the absence of any known tests to determine the loss occa- 
sioned thus we can only say that it is fair to assume it to be 
considerable; also it seems to us that a pitted condenser must 
necessarily place a considerable additional strain on the optical 
properties of the rest of the system, which may result in loss 
of definition of the picture. Of this we are not entirely certain, 
but believe it to be true. 

In any event, bearing in mind the fact that condenser 
lenses are a comparatively cheap commodity, and that any 
injury to the screen result will inevitably result in the sale 
of less seats, we would suggest that it is poor policy to use 
a badly pitted condenser lens. Better throw it away and 
install a lens that you know is all right. 

THICKNESS OF LENSES.— Projectionists will have ob- 
served that some condenser lenses have a very thick edge, 
and some a rather thin edge. The condenser lens should by 
all means be standardized in these dimensions. It is neces- 
sary that a condenser lens edge have at least l/16th of an 
inch thickness, because if it were brought right down to a 
sharp edge the tendency to breakage would be greatly 
increased. l/16th of an inch is, however, ample, insofar as 
prevention of breakage be concerned, and since additional 
thickness of a glass tends in any event to absorb light 
energy to some extent, especially if the glass be of poor 
quality, it therefore follows that condenser lenses should 
have a standard edge thickness of l/16th of an inch, thus 
minimizing the thickness of the lens. 

In Fig. 37 we see at A a lens having unnecessary edge 
thickness, and at B a lens having the correct thickness of 
edge. Condenser lenses should have standard 4^2 inch 
diameter — not pretty nearly but exactly. Another point is 
that the more nearly lens edge thickness and diameter be 
exactly standardized the more nearly will projector manu- 
facturers be able to make condenser mounts which will 
properly receive and support the lens. 



CONDENSER LENS SURFACE.— It is highly essential to 
even screen illumination and efficiency of service that con- 
denser lenses be ground to a perfectly true surface and well 

In Fig. 38 we see a condenser over which is placed a metal 
plate in which are drilled two small holes near the edge of 
the lens. The resultant rays pass through the aperture, as 
will be seen, and are by the condenser directed straight 
through to spots on the screen. 
In Fig. 39 we see the same thing exactly, except that the 

projection lens 
has been 
placed in po- 
sition. These 
are actual 
phot o graphs 
taken of light 
rays, exactly 
as shown. A 
study of them 
will, we be- 
1 i e v e, con- 
vince even the 
most skeptical 
that the con- 
denser actual- 
ly directs or 
projects the 
ray forward in 
a straight line 
toward the 
spot, aperture 
and projection 
lens. Now if 
the converg- 
ing lens has 
not a perfect- 
ly true sur- 
face it natur- 
a 1 1 y follows 
that instead 
of the rays be- 
ing all direct- 
Figure 37. e d forward 


symmetrically to their proper place, the refraction will be 
uneven, the resultant illuminaton of the film will also be un- 
even, and since the projection lens receives the image of the 
film exactly as it is illuminated by the condenser, it follows 
that the screen itself will be unevenly illuminated. 

We learn from this the importance of having a condenser 
with a ground surface of perfectly true curvature. 

The condenser lens which has poor polish will be a source 
of continual loss, because the poorly polished surface will 

Figure 38. 

reflect a greater percentage of the light than will one with 
high polish. Optic Projection, page 602, paragraph 841, says : 

The polished surface of a lens reflects some light about 4 
per cent, or 5 per cent, at each surface between glass and air; 
8 per cent, to 10 per cent, for each lens or plate of glass. If 
the surface of the glass be not perfectly clean or perfectly 
polished the light losses may amount to much more, sometimes 
15 per cent, at each surface. 

We thus see the great importance of having not only per- 
fectly clean lenses but lenses with highly polished surfaces. 

TYPES OF CONDENSER.— For motion picture projection, 
where the electric arc is the light source, there are three types 
of condenser (other than the curved mirror used with the reflec- 
tor type arc lamp, which really is a condenser, though we do 
not call it that) in use in the United States of America and 
Canadian America. 

They are (A) the plano-convex, consisting of two plano- 
convex lenses, which in practice should be spaced with the 
crests of their curved surfaces not to exceed one-sixteenth 
of an inch apart, (B) the Cinephor parabolic, which consists of 
a plano-convex collector lens of suitable focal length for the 
work in hand, and a parabolic converging lens of fixed, unvary- 
ing focal length and (C) the miniscus bi-convex condenser, which 
is very little used since the Cinephor parabolic was put out 
by the Bausch and Lomb Optical Company. Its only advan- 
tage over the plano-convex condenser is that it gives a longer 
distance from the optical center of the condenser to the film 
("Distance Y") for any given arc distance, than did the plano- 
convex, hence it possessed advantages when the projection 
lens working distance was long. It consists of a meniscus 


collector lens and a bi-convex (convex on 
both sides) converging lens. 

The plano-convex is the one most in use. 
It is used with the convex sides of both 
lenses next each other, which has the ad- 
vantage of producing somewhat less spheri- 
cal aberration, and a somewhat less loss by 
reflection from the surfaces of the lenses. 
Aside from this there is no difference in 
results when the flat sides of the lenses are 
placed together. 

The Cinephor parabolic condenser (see 
page 212a) is a condenser which is in large 
measure corrected for spherical aberration. 
It has a decided advantage when the projec- 
tion lens working distance is long; also where 
there is a heavy projection angle, since it 
permits of using a projection lens of smaller 
free diameter, and as lens diameter is re- 
duced, other things being equal, depth of 
focus is increased, which means that the 
lens will have increased power to focus 
things at different distances from it. 

The advantage of this is understood when 
we consider that where there is a heavy angle 
of projection the top of the screen is much 
nearer the lens than is its bottom. 

Under modern conditions we may discard 
everything except the plano-convex and the 
Cinephor parabolic condenser for arc light 
projection — except, of course, where the re- 
flector type arc lamp is used. There is no 
longer need for the meniscus bi-convex con- 


No matter what your plano-convex con- 
denser combination may be, the lenses should 
always be spaced so that the nearest sur- 
faces are as close together as you can get 
them without actual contact — say l/16th of an 
inch apart if you are able to get them that 
close. Optically the best possible position 
would be with the surfaces actually in con- 
tact, but this would communicate heat from 
the collector lens to the converging lens by Figure 39* 

mechanical means, which would be highly 
objectionable. The lenses should be just sufficiently separated 
to avoid this. 
Lens tables are calculated on the equivalent focus of con- 


denser lenses set with their nearest point of contact sepa- 
rated not to exceed l/16th of an inch, and spacing the lenses 
further apart has the effect of altering the equivalent focus 

Figure 40. 

of the combination, besides setting up additional and un- 
necessary loss as follows : 

lector lens, the one next the light source, does not send for- 
ward a parallel beam when the arc is in correct position for 

In Fig. 40, we see the rays sent forward by a 6.5 inch focal 
length collector lens, when the arc is in projection position. 

Now stop and consider for a moment. . Examining Fig. 40, 
it will readily be seen that loss of light will occur between 
the lenses of the condenser, and the farther away the con- 
verging lens is from the collector lens — the greater the sepa- 
ration of the two lenses — the greater the loss will be because 
of the fact that the outer margin of the beam sent forward by 
the collector lens falls outside the converging lens. 

The projectionist has only to observe the ring of light 
surrounding the converging lens of his own projector to under- 
stand the truth of this. 

CONDENSER MOUNTS.— Most of the late types of pro- 
fessional projectors have very good condenser mounts. A 
condenser mount should have the following points of excel- 
lence : (a) accessibility, in a way that will allow the pro- 
jectionist to readily get at the lenses and to remove a hot 
lens with a minimum effort and in a minimum space of time ; 
(b) the lens should be in firm contact with the metal of the 
holder around its entire edge, and the lens holder should be 
so calculated that it will act as a heat equalizer, causing the 
thin edge of the lens to heat up and cool down as nearly 
as possible at the same speed as does the thick centre of 


the lens ; (c) the lens holders should be so made that the 
distance between them may be altered quickly and con- 
veniently from the outside of the casing, in order to properly 
adjust the distance between lenses of varying thickness, or 
to adjust them for other reasons; (d) the condenser holder 
should be strongly and substantially made, and when in 
projection position should be locked firmly in place; (e) 
modern methods favor the placing of the condenser inside the 
lamp house, which is, in our opinion, the better practice, 
since it subjects the whole condenser to the even, though 
high, temperature of the lamp house interior. 

CONDENSER BREAKAGE.— Often this is a difficult thing 
to deal with. Cases of excessive breakage have come under 
the observation of the author for which there was, apparently, 
no reason. As a general proposition, however, always pro- 
vided the condenser lenses be properly mounted (see con- 
denser holders, page 365) excessive breakage will be found 
to be due to one of three things, viz : (a) an improperly 
selected or improperly adjusted optical system which places 
the arc too near the collector lens ; (b) excessive heat in the 
lamp house due to improper ventilation thereof (see lamp house 
ventilation, page 362) ; (c) excessive flaming of the carbons. 

WHAT THE CONDENSER DOES.— The condenser lens 
acts, in a way, the same as does a photographic lens. It forms 
an image of the floor of the carbon crater, or of the Mazda 
filament if a Mazda light source be used, though the whole 
arc crater is not in focus at any one plane, because various 
points of its surface are at varying distances from a plane 
established at the optical centre of the condenser. That the 
floor of the carbon arc crater is not of even luminosity has 
been amply proven, and is now an accepted fact. This is due 
to the fact that in order to have absolute evenness of luminos- 
ity it would be necessary that the current flow be exactly the 
same in every point of the crater floor, and this would neces- 
sitate the entire crater floor area having precisely the same 
conductivity, or perhaps resistance would be the better word, 
at every point, a thing too perfect to expect in a commercial 
product such as carbon ; moreover, the core of the carbon 
usually offers a different degree of luminosity than does the 
floor of the crater surrounding it. 


It therefore follows that if it were possible for the con- 
denser to focus the entire crater floor sharply at the aper- 
ture, then the aperture area, and therefore toe film photo- 
graph, would be unevenly illuminated, and since the film plane 
is one of the conjugate foci points of the projection lens, and 
the screen the other, the screen itself would be unevenly 

From this it is seen that it is important that the actual 
image of the crater be somewhat broken up ,and not focused 
sharply, as a whole, at the film plane. This is in large meas- 
ure accomplished because of the fact that different points of 
the crater are different distances from a plane established at 
the optical center of the condenser, or to make it more 
simple, from the plane of the face of the collector lens, which 
is different, but for our purpose amounts to the same thing. 
This is especially true of the ordinary arc, the crater of which 
sets at an angle to the face of the collector lens, and is to 
some extent true even .of the high intensity arc, because its 
crater is deep and concave in form; also the ball of vapor 
from whence comes most of the high intensity light, is round 
in form, or at least is presumed to be so. The further fact 
that the plano-convex condenser has a great deal of spherical 
aberration helps to "break up" the crater image, and thus 
produce evenness of illumination at the film plane. 

LOCATION OF CRATER IMAGE.,— In the stereopticon the 
image of the crater is focused within the projection lens itself. 
Theoretically this would be the proper place to focus the 
crater in motion picture projection, but in practice it is found 
that where the light source is a carbon crater and the con- 
denser a plano-convex this is not practical. It is difficult, if 
not impossible, to get evenness of screen illumination with the 
main point of focus of the crater too near the aperture. The 
practice which generally gives good results is to focus the 
center of the crater slightly on one side or the other of the 
film plane, but if an attempt is made to focus the crater 
image too far on the projection lens side of the film plane, 
the ghost zone of the plano-convex condenser beam will be 
encountered (see page 177). 

This point has caused a great deal of discussion and dis- 
sension among those striving to solve the problem of the 



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optical train of the motion picture projector. Theoretical 
opticians for a long while insisted on basing their calculations 
for the projector optical train on a crater focused either 
within the projection lens itself, or else far on the screen side 
of the film plane. For years this was a serious stumbling 
block, for it just would not work out in practice, and until' 
concessions were made which enabled theory and practice to 
harmonize, little real progress was made. 

SHAPE OF SPOT. — The projectionist using an ordinary 
arc and plano-convex condenser should observe the shape of 
his "spot," since therein he will find a direct indication of the 
condition of his crater with relation to the lens. The spot is 
a somewhat out-of-focus image of the light source, which is, 
in the case of the ordinary arc, the floor of its crater. A 
round, well defined spot is an indication that the crater is 
itself of good form, and that its angle to the face of tht 
collector lens is at least fairly good. 

SIZE OF SPOT. — When we consider the size of the spol 
at the aperture of the projector we find it a subject full of 
complications. About the smallest spot that can be carried 
with an assurance of maintaining a clear screen, when using 
the ordinary arc, is 1^2 inches in diameter. The modern 
projectionist, however, as a general proposition prefers the 

175 inch-diameter spot, since with the smaller one there is 
always the chance for the appearance of discoloration of the 
light on the screen, a thing which must at any cost be avoided. 

Fig. 41 is the diagrammatic representation of the possibili- 
ties for loss of light through unnecessary enlargement of the 
spot. This holds good regardless of the kind of light source 
used. With a spot 1.5 inches in diameter, only 43 per cent, 
of its area covers the aperture, the rest being intercepted by 
the cooling plate. We thus see that with a spot of medium 
practical diameter, 57 per cent, of its area is intercepted by 
the cooling plate. If we increase the size of the spot to 2.75 
inches diameter, then the aperture will only cover 13 per cent, 
of the area of the spot, 87 per cent, of which will be inter- 
cepted by the cooling plate. 

The obvious lesson of this is that an unnecessarily large 
spot is an enormous waster of light, hence of electric energy. 


When using ordinary or high intensity arcs, the main point 
in deciding upon the focal length of the condenser, is to secure 
a focal length which will establish the crater produced by the 
amperage to be used at the minimum practical distance from 
the collector lens, at the same time giving the smallest prac- 
tical diameter of spot and a maximum distance from the face 
of the converging lens to the aperture. (See page 162.) 

The latter, however, is only of importance in cases where, 
due to the large diameter crater produced by high amperage, 
it is difficult to secure sufficient distance between the face of 
the converging lens and the aperture to confine the divergence 
of the beam beyond the aperture within limits which will enable 
the projection lens to pick up the entire beam, and at the same 
time keep the spot at the minimum practical working diameter. 

In considering the relation of the focal length of the con- 
denser to the size of the spot, to the distance from the 
center of the condenser to the spot, to the size of the crater, 
and to the distance of the crater from the center of the con- 
denser, let it be clearly understood that while the office of 
the condenser is to intercept as much as possible of the light 
emerging from the crater, and to concentrate it on the 
aperture, it after all, in so doing acts precisely the same as 
would a photographic lens. With a condenser of given focal 
length, and a crater of given diameter, the diameter of the 
spot will be as many times the diameter of the crater as the 
distance from the apex of the curved surface of the collector 
lens to the floor of the crater is contained times into the 
distance from the apex of the curved surface of the con- 
verging (front) lens to the aperture, when the curved sur- 
faces of the lenses are not to exceed l/16th of an inch apart. 
For instance : If the distance from the apex of the curved 
surface of the collector lens to the aperture is 16 inches, the 
distance from the apex of the curved surface of the col- 
lector lens from the floor of the crater 4 inches, and the 
crater is .5 of an inch in horizontal diameter, then the re- 
sultant spot will be 16 -f- 4 = 4, and 4x ^ inch = 2 inches 
in diameter. This applies equally for both plano-convex and 
meniscus bi-convex lenses, except that when figuring the 
meniscus bi-convex, instead of figuring from the curved sur- 
face to the floor of the crater we figure the distance from a 
point % of an inch in front of the convex face of the meniscus 
lens, and the other distance from the center of the bi-convex 
to the aperture. 



With the foregoing in mind it readily will be seen that the 
necessary enlargement of the crater image at the spot will 
depend on the number of amperes we are using, since the 
diameter of the crater increases in proportion to the number 
of amperes used. (See Page 394.) The distance from cra- 
ter to apex of curved surface of collector lens, and from the 
apex of curved surface of converging lens to film, are known 
respectively as "distance X" and "distance Y." The points 
from which they start are shown in Fig. 42. 

The average pro- 
jectionist who reduces 
his spot diameter by 
pulling his lamp back 
a greater distance 
from the face of the 
collector lens is more 
than likely to discred- 
it the foregoing, be- 
cause, while he has 
decreased his spot 
diameter, he has 
either not increased 
the screen illumina- 
tion at all, or to no 
appreciable extent. 

The fact is, he has 
not acted intelligently. 

He should have installed a shorter focal length converging lens 
in order to maintain his arc-to-lens distance at minimum, at 
the same time setting up a condition which would cause a 
heavier convergence of the beam on the other side of the lens. 

Of course we realize that intelligent work in this direction 
is hampered by difficulty in securing lenses of necessary 
focal lengths, but much may be accomplished, nevertheless, 
and the whole matter forms an interesting and profitable 
field for experiment and study on the part of projectionists 
and engineers. 

The problem is to keep distance X the minimum possible, 
without setting up excessive lens breakage, at the same time 
maintaining a maximum distance and standard diameter spot. 

In this connection remember that: 

The spot diameter will always be as many times the diame- 
ter of the crater as distance X is contained into distance Y. 


Figure 42. 



SLIDE CARRIER WASTE.— The loss of light caused by 
fixing a slide carrier permanently in place in front of the con- 
denser is illustrated in Fig. 42A. 

Lack of understanding and appreciation of such things 
causes what is literally a tremendous amount of waste of 
light, which means electric energy, when we come to consider 
motion picture theatres as a whole. Each theatre may waste 
but comparatively little, but there are many thousands of 

CRATER IMAGE UPSIDE DOWN.— Due to the fact that 
the condenser reverses the image of the crater in projecting 

it, the image 
AREA OF A'/z" CONDENSER LENS 15.9 SQ.. IN. of the crater 
AREA OF BLACK CIRCLE, *ie" WIDE, 2.54- SQ. IN. is upside down 

OR 16% OF THE TOTAL AREA OF THE LENS. at the Spot. In 

drawn to scale some cases 

where the 
bottom of the 
crater is in 
fairly good 
focus at the 
cooling plate 
it is possible 
to discern the 
image of the 
lower carbon 
tip at the top 
of the cooling 
plate. It may 
often be clear- 
ly seen when 
feeding the 
carbons while 
the arc is 
Figure 42A. ZONE. — The 

plan o-convex 
condenser beam from the ordinary arc carries a well defined 
ghost zone. This is due to both chromatic and spherical 
aberration, which is always present in uncorrected lenses such 
as those used for condensing. 

In Fig. 43 a crater is constructed by cutting an aperture 
in a piece of cardboard and placing over it a bit of ground 
glass, behind which we establish a 100 C. P. incandescent 





Figure 43. 

lamp. The crater and the screen are placed at conjugate 
foci points of the plano-convex condenser, as shown. The 
screen corresponds to the aperture of a projector. We cover 

the front surface of 
the collector lens 
with a piece of 
cardboard in which 
is a pin hole located 
as shown. 

The results as ob- 
served upon the 
screen are: The 
crater is focused in 
full definition, but is 
colored with the 
shades of the spectrum in the manner shown. It has been 
demonstrated by the Kinemacolor process that all the colors 
of the spectrum may be reduced to approximately two shades, 
viz.: A reddish-orange and a bluish-green, which, for the 
sake of clearness, we will call orange-green. 

In Fig. 44 the same conditions are shown as those described 
in connection with Fig. 43, except that the colors of the spec- 
trum have been reduced to the two primary shades, viz. : 
Orange and green. You will note that under this condition at 
the screen (or aperture) the colored rays have combined and 
formed white light. 

Now if the process shown in Fig. 44 be continued, and a 
very large number of rays be drawn, using orange and green 
ink, the result will be as illustrated in Fig. 45, which is 
merely Fig. 44 elongated to more nearly approach actual 
working conditions. 
In Fig. 45 it is ob- 
served that the beam 
is enclosed by an 
orange envelope, 
which is thickest 
toward the central 
part of the beam 
and comes to a 
point, or is not 
present at the aper- 
ture and the condenser. The beam has a core in its center, 
which same is composed of the violet, the blue and the green 
shades of the spectrum. The white part of the beam is caused 


frJ3m.;JL$J m®L~» 

Figure 44. 




by the mixture of the two other primary shades, but the mix- 
ture is not perfect at all positions. At AA Fig. 45 the white 
light is most pure, but as it approaches position BB the colors 
at the faulty end of the spectrum begin to predominate, so 
that at section BB the white zone has changed to a dirty 
purple. Considering this condition we may readily understand 
why a ghost appears when the condenser is brought far enough 
toward the aperture. When the aperture is properly located, 
all the colors of the beam finally combine at that point to form 
pure white light, and all light beyond the aperture is white. It 
must, however, be remembered that the results shown in Fig. 
45 can only be approximately true, since all the colors of the 
spectrum, which are infinite in number, have, for the purpose 
of the experiment, been reduced to two shades. Even if only 

Figure 45. 

seven of the primary colors have been used in the drawing, the 
straight lines in Fig. 45 would show as curves, and more closely 
resemble the true shape of the actual beam. The projectionist 
is not able to see these various zones with the naked eye. The 
fact remains, however, that while invisible to the eye they are 
there, and it is their presence which causes the blue spot to 
appear in the center of the screen when the aperture is brought 
too near the condenser, or the crater is advanced too close to 
the collector lens, which latter has the effect of advancing the 
ghost zone nearer to the aperture. 

PURE LIGHT. — In this connection one of the important 
functions of having the crater as nearly as possible in focus 
at the aperture is to purify the light and avoid color effects. 

The reflector type arc presents a new condition, which will 
be dealt with separately in another place. Before attempt- 
ing the adjustment of the optical train it will be well 
to carefully study and consider all those various things 
which have already been set forth under the general 


heading "Lenses," beginning at page 125. We will provide 
you with the necessary charts and diagrams, by the applica- 
tion of which you will be able to so adjust your projector 
optical system that maximum results will be had, both in 
brilliancy and evenness of screen illumination, and, what is 
almost equally important, they will be had at maximum 

It must be remembered, however, that where local condi- 
tions vary so widely, even the most carefully calculated and 
prepared formula is likely to in some measure fail of its 
highest purpose unless it be applied with intelligence and 
understanding. It is for this reason we urge projectionists 
to study and come to an understanding of the underlying 
principles which govern in the selection and adjustment of 
the projector optical system. 

It is a deplorable fact that the lack of such understanding 
on the part of projectionists, coupled with the failure of 
condenser lens manufacturers to provide proper lenses and 
the failure of theatre managers and exhibitors to purchase 
optical equipment of the right design and quality, has, ever 
since the very inception of motion picture projection, not 
only caused a literally huge waste in light, which means a 
waste in electric energy, but has also made it impossible to 
exactly duplicate the illumination of all points of the film 
photograph upon the screen, without which it is impossible 
to secure the same apparent depth in the picture which the 
natural scene presented to the "eye" of the high grade 
camera lens, and which was by that lens impressed faith- 
fully upon the film. 

It has often been remarked that the same photoplay has 
an apparently increased stereoscopic effect, or perhaps we 
might better say a greater "depth" in one theatre than in 
another, and this has been placed to the credit of the screen, 
whereas the real credit was not due to the screen at all, but 
to the correct selection of the various elements of the optical 
system and their correct adjustment with relation to each 
other, so that the film image was evenly illuminated and the 
evenness of illumination was faithfully transmitted to the 

One of the big outstanding facts in projection is that even- 
ness of illumination is absolutely essential to perfect results 
on the screen, and evenness of illumination at the screen is 
impossible unless we first secure evenness of illumination of 
the film photograph at the aperture, and then project the 


even illumination of the picture at the aperture to the screen 
without change. 

It is a comparatively simple problem to secure at least a 
high degree of evenness of illumination at the aperture, but 
a study of the subject will convince even the most skeptical 
that unless the projection lens receive an equal amount of 
light from every point of the area of the aperture there can- 
not possibly be evenness of illumination at the screen. 

Figure 46. 

If the student will carefully examine Figs. 46, 47 and 49 
and apply their meaning, he will, for one thing, immediately 
know that a projection lens which has not sufficient diameter 
to receive the entire beam from the aperture, cannot possibly 
project evenness of illumination of the film photograph to the 

Fig. 46 is a photograph, in which X is a standard projector 
aperture, mounted on an optical bench, over which has been 
placed a plate of metal in which two minute holes were 
drilled. This aperture is illuminated by a standard spot 
from a regular projector plano-convex combination condenser, 
placed with its center 18 inches from the aperture. 1 is 
the rear combination of the projection lens and 2 the front 
combination, space Y having been sawed out of the lens 
barrel so that we were able to look right into the lens and 
watch the action of the rays. Smoke was blown into the 
light beam, thus making it visible and enabling us to photo- 
graph the results. Absolutely no change has been made in the 
photograph, except that we have drawn in the dividing lines 
between the two light cones which, while quite visible in the 
photograph itself, would not show clearly in the printed reduction. 

Considering Fig 46 you will see that a cone of rays goes 
forward, diverging fan-shaped, toward the projection lens. 
That this is true you ought to know because, since every 
pin point of the film photograph must be refocused on the 



screen in its appropriate place, it follows that the illumina- 
tion it receives must go forward to the projection lens as a 
separate unit, and by the projection lens must be sent for- 
ward to its appropriate spot on the screen. 

Consequently we must treat every pin point of the film 
photograph as an entirely separate proposition from every 
other pin point when we consider the item of illumination, 
and the projection of that illumination to the screen. 

If every portion of the film photograph is evenly and 
equally illuminated it follows that the light from each pin 
point of the film goes forward toward the projection lens 
as a cone-shaped bundle of rays, the base of which is at the 

projection lens. Since the amount of divergence of the rays 
is in exact proportion to the diameter of the condenser and 
its distance from the aperture, the cone starting at the center 
of the film picture is always likely to enter the projection 
lens in its entirety, which means that the full value of the 
illumination passing through that point will be projected for- 
ward to its appointed place on the screen, giving the center of 
the picture on the screen what we may call 100 per cent, 
illumination, appearing to the audience practically the same 
as it appeared to the eye of the camera. 

On the other hand, again considering Fig. 46, suppose one 
of these cones to have its point at the extreme corner of the 
projector aperture. This cone will also diverge equally with 
the one in the center of the picture, and if the projection lens 
has insufficient diameter to receive its entire base, it is 


apparent that a portion of this bundle of rays will fall outside 
the projection lens, hence will not be projected to the screen. 
This, of course, means that that particular point of the film 
photograph will be deficient in illumination, and therefore 
will not have the apparent depth presented by the center of 
the picture. 

That this unevenness of illumination may not be in any 
degree apparent to the eye when the blank screen is viewed 
proves nothing, because the eye is unable to detect uneven- 
ness of illumination under those conditions unless it be very 
great indeed, or unless the weak points be also discolored. 
But it can and does detect the difference in apparent depth 
of the picture, though it does not recognize it as such. In 
fact, it does not recognize it at all, and could not possibly 
recognize it except in case two projections of the picture be 
placed side by side, one evenly illuminated and the other 
not evenly illuminated. The eye would then see that the 
evenly illuminated picture was much more pleasing than the 
one which was unevenly illuminated. 

In Fig. 47 we have diagrammatic representation of what 
we have been talking about with relation to Fig. 46. A is a 
standard aperture in which dots X and Y represent respec- 
tively two pin points in the film photograph, one in the cen- 
ter and one in the upper left hand corner. B is a line 
representing a side view of the same aperture. C is the back 
factor of a 1^-inch-diameter projection lens, at 4-inch work- 
ing distance. Cones D, E, F and G are laid out exactly as 
they would be with a standard diameter condenser converg- 
ing lens located 18 inches from aperture B. Dotted line 
H — H represents the diameter of projection lens C. In other 
words, lines H — H represent the diameter of lens C at any 
point within their length, and we may readily see that in 
order to take in the entire cone D — E, the lens would neces- 
sarily have to be moved towards the aperture to point J. 
Dotted lines I — I represent the diameter projection lens C 
would have to have in order to accommodate cone D — E in 
its entirety. 

It seems hardly necessary to point out the very obvious 
fact that under this condition the center of the screen image 
must and will, in the very nature of things, be better 
illuminated than will the outer margins. In other words, 
under such a condition it is utterly impossible to have an 
evenly illuminated picture on the screen. 




beam does diverge beyond the aperture (when using plano- 
convex condenser and ordinary or high intensity arc) is amply 
proven in Fig. 48, which is the photograph of a standard pro- 
jector aperture illuminated by a condenser, exactly as in prac- 
tical projection. By laying a straight edge along the upper 
line of the diverging ray it will be found to just touch the 
lower edge of the condenser, and if the straight edge be laid 
on the lower edge of the light ray it will just touch the upper 
edge of the condenser, proving that the divergence of the ray 

Figure 48. 

is in exact proportion to the diameter of the condenser and its 
distance from the aperture, the size of the projector aperture 
being always standard. 

In Fig. 49 we have the photographic representation of what 
happens when the projector lens has insufficient diameter to 
receive the entire light beam. This not only results in serious 
loss of light, but, as has already been explained, causes un- 
evenness of illumination on the screen and detracts seriously 
from the depth of the picture. 

DISTANCE. — Fig. 50 shows the effect of distance of plano- 
convex condenser from aperture on projection lens diam- 
eter. In this drawing we have a plano-convex condenser 
with the face of the converging lens located 16 inches 
from the aperture. The broken lines indicate the divergence 



of the beam at the plane of the pro- 
jection lens, which is presumed to be 
5 inches from the aperture — a 5-inch 
working distance. We have another 
condenser lens, of equal diameter, lo- 
cated 21 inches from the aperture, the 
divergence of the beam from which 
is indicated by the solid lines. We 
thus see that the removal of the con- 
denser to a greater distance from the 
aperture has the effect of narrowing 
the divergence of the beam beyond 
the aperture, thus permitting the use 
of a projection lens of smaller diam- 
eter. But let it be clearly understood 
that as we increase the distance from 
condenser to aperture it will be neces- 
sary to install longer focal length 
condenser lenses in order to maintain 
the size of the spot (assuming, of 
course, that the crater was in the first 
instance, the minimum distance from 
the face of the collector lens for 
the amperage being used), and the 
installation of a longer focal length 
condenser means that the crater will 
be farther away from the plane of the 
collector lens. This automatically sets 
up light loss (see Fig 36H), in that 
it reduces the amount of light flux 
falling upon the surface of the collec- 
tor lens, but if the projection lens was 
of too small diameter to pick up the 
ray, the increased amount of light it 
will receive by reason of the narrow- 
ing of the divergence of the ray will 
more than compensate for the loss at 
the condenser. 

CAUTION.— This is one of the 
things which must be applied with 
.Figure 49. understanding, and the more thor- 

oughly the projectionist understands 
all the various propositions involved the better result he will 
be able to attain, both in screen illumination and efficiency. 





1 ' Hi 








I \ 


1 ) 


\ I 

/J / 

\ \ 


Figure 50. 



uncorrected condenser, such as, for example, a plano-convex 
combination, the condenser beam will spread out, or diverge, 
on the projection lens side of the aperture, as per Figs. 48, 
49, 50 and 51. 

This divergence or spreading of the beam will be in exact 
proportion to (A) the free diameter of the converging lens of 
the condenser, and (B) the distance of the face of the con- 
verging lens from the aperture. 

This is diagrammatically illustrated in Fig. 50, in which the 
solid lines represent the outline of the beam beyond the aperture 
when the face of the converging lens is 16 inches from the 



murm n*TE 

Figure 51. 

aperture, and the solid lines when the distance is 21 inches. 
The same thing is photographically illustrated in Fig. 38. 

In proof of the possibilities for literally enormous loss of 
light through divergence of the beam beyond the aperture I 
present to you Figs. 52, 53 and 54. I might also show other 
variations of the same thing, since the range of divergence 
conditions is very wide, but these should serve to give you an 
understanding of the matter, which is all that is really neces- 


sary. The charts show actual photometric measurements of a 
screen located as indicated. The light source used was com- 
paratively weak, but the ratios would be exactly the same were 
a more powerful illuminant used. 

In each chart, observe a heavy black circle. This circle 
represents a two-inch diameter projection lens, and shows 
exactly what proportion of the total area of the light beam that 
size lens would cover under the condition shown. The measure- 
ments were made of the cross section of the beam, looking 
TOWARD the condenser, at a distance of three and seven 
inches from the aperture. 

In Fig. 52 the reading is at seven inches from the aperture, 
on the projection lens side of it, of course, with the face of 
the converging lens located ten inches from the aperture. The 
screen is at position C, Fig. 51. 

Examining the chart and the tabulated figures below it you 
will observe that w r hereas the foot candle brilliancy (the figures 
in each section indicate foot candles) is much higher in zones 
1, 2 and 3, which fall entirely within the lens, than in the zones 
which fall outside the lens, still the total area of the outer 
zones is very much the greater of the two, hence while its 
u%it area illumination is comparatively low, it nevertheless 
carries a large percentage of the total of the entire beam. 

Sunlight may be just fairly bright as you view it, but, as 
you know, if you use a two-inch diameter lens and concentrate 
the rays within that just-well-illuminated circle to a spot a 
quarter of an inch in diameter, you will find that there really 
is a rather great amount of light in the total area of the two- 
inch circle. The same identical thing applies here. All the 
light in the outer zones is wasted, because it cannot enter the 
lens. It is weak by comparison with the unit area illumination 





V * — -J 


x 8a, 


• A Vr; 



P fi»fK 









06 X 




















f k">f & 






Figure 52. 

Reduced by 1.25 inches, size of beam 4^x4". 



of the inner zones, but if it were concentrated to a spot you 
might be astounded at the total amount of it, nevertheless. 

In Fig. 53 we have exactly the same condition as in Fig. 52, 
with measurements made at position C, Fig. 51, but the face 
of the converging condenser lens has been pulled back until 
there is 18 inches between it and the aperture. 

Observe the effect on size of the beam at position C by this 

Seven Inches from Aperture, with Con- 
denser 18 Inches from Aperture. 















36. o 





4 a. 6 


/!//)? 29 






Figure 53. 



condenser movement. Of course, Fig. 52 represents an 
abnormal condition, and one never actually found in practice. 
It was used merely to show a rather startling amount of light 
loss, and thus concentrate your attention on the matter. 

The condition shown in figure 53 is very often encountered 
in actual practice, however, and the loss of light is very great ; 
also there are other evil effects. 

Exact Size and Shape of Ray Three 
Inches from Aperture When Con- 
denser Is 18 Inches from Aperture. 








ay. 3 














Figure 54. 

In Fig. 54 we have the face of the converging lens still 
18 inches from the aperture, but the measurements are made 


at position A, Fig. 51. You will observe that, with the face 
of the converging lens 18 inches from the aperture and the 
two-inch free diameter projection lens three inches from the 
aperture, the light would, to all intents and purposes, be all 
admitted to the projection lens, and the condition would be 
almost 100 per cent, perfect. 

If, however, the working distance of the projection lens 
(the distance of its rear surface from the film) were increased, 
light loss would immediately be set up unless (A) the con- 
denser be pulled further back or (B) a larger diameter 
projection lens be substituted for the two-inch one. 

However, let us remark, in passing, that condenser distance, 
commonly called "Distance Y," cannot be altered at will 
without changing the arc distance — distance light source to 
face of converging lens — which usually is inadvisable if it 
alters the correct distance (usually the least distance, in the 
case of the arc, at which the arc can be operated without undue 
lens breakage), since if it shortens it there will be excessive 
lens breakage, and if it lengthens it, then the effect set forth 
in Fig. 36H is made operative, and sets up heavy light loss. 

LENS CHART. — In past editions we have published a lens 
chart. This we now think inadvisable, for the reason that a 
most excellent chart has been worked out by John Griffith. 
This chart is far in advance of anything heretofore published. 
It can be incorporated in this book only in very reduced 
size. It may be had size 14" x 20" from the Chalmers Pub. 
Co., 516 Fifth Avenue, New York City. Price $1.00. We 
earnestly advise every theatre manager to get one, have it 
put in a frame under glass, and place it either in the projection 
room or in the office, where it will be available for consultation 
at all times. It is worth far more than its price to any theatre 



using ordinary or high intensity arcs for projection. It is NOT 
for Mazda or the reflector type arc. 

WARNING. — Projectionists and exhibitors are warned that 
unless the various elements of the projector optical system be 
intelligently selected, and carefully and intelligently adjusted 
with relation to each other, the best results will certainly not 
be had on the screen, and the results may be very poor indeed, 
compared to what they should be and would be were correct 
procedure followed. There may be either loss of light, poor 
quality of light, unevenness of screen illumination and injury 
to the depth, or stereoscopic effect set up, or there may be 
all of them. 







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It borders on the absurd for a projectionist to be without so 
valuable a thing as the Griffith lens chart, when it may be had 
for so small a sum. If the projectionist does not own one, the 
theatre itself should get one. 

58 we present a very valuable diagram by the careful, in- 
telligent application of which the projectionist using high 
amperage will be enabled to get both the correct condenser 
distance for his objective and the correct focal length con- 
denser to maintain the proper size spot at that distance. 
This diagram must, however, be used with caution, because 
there may be a tendency to apply it in cases where the in- 
stallation of a projection lens of larger diameter would serve 
better. In other words, it would be bad practice to use this 
diagram for the purpose of accommodating a projection lens 
of too small diameter for the work, since if this be done, 
although there might be some gain as against the former 
condition, still the ultimate result will not be what it should 
be, and would be if a projection lens of proper diameter were 
used. The method of applying the diagram is as follows : 
First measure both the exact working distance of your 
projection lens and the exact free diameter of its aperture. 
Next examine Table No. 8 and in the left hand column find 
the number corresponding to, or most nearly corresponding 
to, the free diameter of your projection lens. Now run your 
finger out along the horizontal row of figures opposite the 
free diameter of your projection lens until you find the 
figure most nearly corresponding to its working distance, 
and at the top of that column will be the distance the center 
(point midway between the lenses) of the condenser must 
be from the aperture in order to allow the entire light beam 
to enter the projection lens. 

For example : Suppose the working distance to be 5 inches, 
and the projection lens aperture 2.5 inches. In the 15th 
space from the top in the left hand column of Table No. 8 
we find 2.5, and in the 11th column to its right we find 5. At 
the top of column 11 we find 18, which means that under the 
condition of working distance and lens diameter given, we 
must have 18 inches from a point midway between the two 


condenser lenses to the aperture in order to get the entire 
light beam into the projection lens. 

If the working distance had been 5.5 instead of 5, then the 
right distance would be 18.5 inches, because the next working 
distance given opposite the 2.25 lens diameter is 5.437, which 
is practically 5.5 inches, and at the top of this column we 
find 19 inches. Since 5.25 is half way between 5 and 5.5 we 
would split the difference between 18 and 19, which would be 
18.5 inches. This would, however, have but slight effect, 
since when we get back as far as 18 inches, an extra half 
inch further alters the angle but very slightly. Nevertheless, 
while we are doing the job we might as well do it with pre- 
cision, therefore under that condition 18.5 would be the 

CAUTION. — And now let us caution you that right here 
judgment and common sense enter. If your projection lens 
has an aperture of 2.25 inches or less, and it is necessary to 
retard the condenser to a greater distance than say 18 or 19 
inches in order to get the entire beam into it, it would be 
better practice to get a lens of larger diameter, because re- 
tarding the condenser calls for a condenser of longer focal 
length (E. F.), which operates automatically to place the 
crater further away from the collector lens, thus weakening 
the light collecting power of the collector lens, or in other 
words the total light available to the spot, see Fig. 36-H, page 

It is therefore for the projectionist to determine when and 
where retarding the condenser should cease in favor of in- 
creasing the projection lens diameter. 

Of course where the projection lens is already of maximum 
diameter, there is then the choice of retarding the condenser 
or reducing its free opening because we must secure evenness 
of illumination at the screen, even though there be actual 
light loss in the process. 

Personally we believe that up to say 18 inches it is better 
to increase condenser distance than lens diameter, especially 
if the projector is considerably out of center with the screen, 
provided the projection lens free aperture be already not less 
than 2 inches, but if the beam cannot be made to enter a 
2-inch aperture lens with a distance of 18 inches, then it is 
better to seure a projection lens of larger aperture than to 
set the condenser as *far back as may be necessary to get 
all the beam into the lens. In this we of course assume that 
projection lenses of desired diameters can be had, which is 







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not always the case, since such lenses are not made in 
aperture diameters greater than twice their focal length of 
the lens. Thus: A 4-inch E. F. projection lens cannot be had 
in greater diameter than 2 inches, and in practice it is doubt- 
ful if you could get one of quite that aperture. P. 139 and 148. 

FOREWORD AND EXPLANATION. In what follows the 
projectionist may not, and probably without this explanation 
would not understand why he is directed to first make diagram 
58 according to the diameter of his present projection lens 
and then, after applying diagram- 58 A, to again apply dia- 
gram 58, using the diameter of the new lens as its foundation. 
He will most likely ask why he could not as well apply dia- 
gram 58 A at once, and then apply diagram 58 without bother- 
ing with the first application of diagram 58 at all. 

The reason is just this: The available projection lenses are 
strictly limited as to diameters, and it is not at all certain that 
the projectionist will be able to get the diameter he should 
have. If the projectionist were to find by application of 
diagram 58 A and table No. 8 that a lens of the required 
diameter cannot be had, he is stuck, or will probably think he 
is, but if he has first applied diagram 58 he knows what the 
line-up for his optical system AS IT ALREADY IS should 
be, and it is important that he have the correct adjustment 
and correct condenser combination even though he cannot 
have the correct projection lens diameter. 

The first application of diagram 58 is to show what con- 
denser combination, and what distance from condenser to 
aperture you should have with the projection lens you are 

You then apply diagram 58 A and table 8 to find what 
diameter projection lens you must have to work efficiently, 
and then, when you have secured the right lens (if you are 
able to), you apply diagram 58 again to see what condenser 
combination and what distance you should have from con- 
denser to aperture for the new lens. 

With this explanation let us proceed with instructions for 

APPLYING DIAGRAM Figure 58. By means of table No. 
8 we have shown you how to ascertain what distance is neces- 
sary between center of condenser (point midway between the 
two lenses) and aperture in order that your projection lens 
admit the entire beam of light from the aperture. Let us now 
go a step further and, by means of Fig. 58 and 58A, exam- 



ine and apply what amouts to a universal method for selecting 
condenser of proper focal length, correct distance center of 
condenser (point midway between lenses) and proper pro- 
jection lens diameter. 

In Fig. 58 you are to draw each line in its alphabetical 
order, and make each measurement in its numerical order. 

First, on a perfectly smooth, flat table top, spread a sheet 
of heavy paper suitable to make a drawing on. Light colored 
wrapping paper, which may be had almost at any store, will 
do very well, though it will be well to get a kind which will 
not tear too easily, because you will probaby wish to retain 
the drawing as a permanent, or at least a semi-permanent 
part of your projection room records. Fasten the paper down 

Figure 58 

securely, preferably with draftsman's thumb tacks, though of 
course ordinary tacks will do. 

Have the following tools ready: A perfectly straight 
straight edge not less than two feet long. (A) An ordinary 
carpenter's steel square will do nicely. (B) A pencil sharpened 
to a long, slim, flat point. (C) A draftsman's triangle for the 
purpose of getting lines exactly at right angles to each other, 
though a carpenter's steel square will serve very well, and 
two such squares will be excellent. (D) A ruler — the square 
will serve, if you have one. (E) An ordinary pair of com- 
passes, such as draftsmen use or such as carpenters use. 
You can make one that will serve very well with two sticks 
fastened together at one end with a screw and a pin or needle 
fastened to each of the other ends. 

FIRST.— Draw line A, Fig. 58. 

SECOND. — Draw line B, Fig. 58, exactly at right angles to 
line A, and near its right hand end. 

THIRD. — Measure distance from your projector aperture at 
film plane (position occupied by film) to first surface of pro- 


jection lens — in other words the working distance — when the 
lens is so adjusted that the picture is in sharp focus on the 
screen, and at that exact distance from- line B draw line C, 
also at exact right angles to line A. 

FOURTH. — Measure the exact free diameter, or opening of 
your projection lens, and at a point on line B distant from 
line A the exact diameter of the opening of your projection 
lens, make a mark. This mark is indicated by the lower point 
of arrow 2, Fig. 58. 

FIFTH. — On line C make a dot one inch below line A. This 
measurement is indicated by arrow No. 3. 

SIXTH. — Through these two points draw line D, as shown. 

SEVENTH. — Draw line E at right angles to line A, and at 
the point where lines A and D are exactly 3.75 (3%) inches 
apart, measured vertically. This measurement is indicated by 
arrow No. 4. 

EIGHTH. — \% inches above and 1% inches below line A 
make a mark on line E. These measurements are indicated 
by arrows Nos. 5 and 6. 

NINTH. — On line C make a mark 1.25 (1%) inches below 
line A. This measurement is indicated by arrow No. 7. 

TENTH. — Draw lines F and G, as per diagram, joining the 
intersection of lines A and E and the lower end of arrow 7 
and the intersection of lines A and C and the lower end of 
arrow No. 6. 

ELEVENTH. — Measure the narrowest diameter of your 
positive carbon crater and, having set your compass to that 
measurement (you may do it with a ruler, but a compass is 
much better), find the point on line A where the vertical dis- 
tance between lines A and G is exactly equal to the crater 
diameter and make a mark on line A at that point, drawing 
line H from the point on line E at upper end of arrow No. 5 
through the point on line A and on across lines G and F. 

TWELFTH. — The distance from line E to the crossing point 
of lines H and F will be the focal length of each condenser 
lens required. For instance: If the distance from line E to 
the crossing point of lines H and F be 7.5 inches, then two 
7.5 piano convex lenses will be required. 

NOTE. — The combination may, and should be modified as 
follows : Use a 6.5 inch focal length collector lens and add 
the difference to the focal length of the converging lens, thus : 
Suppose the diagram calls for two 7.5 inch lenses. We install 
a 6.5 piano convex collector lens, which is one inch shorter 
focal length than is demanded. We then add one inch to the 
focal length of the converging lens, installing an 8.5 instead 


of 7.5. Our condenser then is a 6.5 collector and an 8.5 con- 
verging lens. The E.F. of a 6.5 and an 8.5 lens is the same as 
that of two 7.5 lenses. 

THIRTEENTH.— The distance from line E to line C will 
be the correct distance from center of condenser combination 
(point midway between the two lenses) to projector aperture. 

FOURTEENTH. IMPORTANT.— It must be understood 
that the foregoing is the correct line-up for the projection 
lens you have in actual use at the time, and that the condition 
may or may not be efficient. YOU MUST NEXT determine 
what would be the correct (efficient) projection lens diameter 
under the existing conditions. To do this make a diagram as 
per Fig. 58 A, which you should keep as a permanent part 
of the projection room equipment. 


%o n /£ n /6 is /y /3 /a // /</• 

Figure 58 A 

Line A, Fig. 58 A must be perfectly straight, and exactly 
ten inches long. Divide it into inch lengths, by means of dots 
on or lines drawn across line A, and number the divisions from 
10 to 20, as shown, the 20 being always at the left hand end 
of the line. 

Next draw line B, its left hand end exactly % inch from 
line B and the right hand ends spaced exactly $i of an inch. 

Now select one of your positive craters which is of average 
size, and measure its exact diameter the most narrow way. 
Set a compass with its points spaced apart the exact distance 
equal to the crater diameter (compass is not really necessary, 
but by using' it you will get more accurate results) and find 
the point at which lines A and B, Fig. 58 A, are spaced exactly 
that distance apart. The distance indicated is used in con- 
junction with table No. 8, as follows : 

Let us assume that the crater diameter fits the spacing of 
lines A and B at a point exactly over 16. We find the column 
in table 8 which is headed "16," glance down it until our work- 
ing distance is reached, and then to the left hand column to 
get the correct lens diameter. 

The next move is to purchase a new projection lens, or 
lenses, as nearly as possible to the diameter indicated by 
table No. 8. Next make an entirely new diagram as per 
Fig. 58, using the diameter of the free opening of the new 


projection lens for the measurement indicated by lower point 
of arrow No. 2, Fig. 58. Otherwise the original instructions 
for making diagram Fig. 58 are not in any way changed. 

NOTE. — If it is not possible to secure a projection lens of 
the diameter indicated by table No. 8, it must be remembered 
that if your present lens is too small, then any increase in 
diameter will result in a more economical operation in that 
it will cause the transmission of a greater percentage of the 
light to the screen and will give a more even illumination of 
the screen, hence the effect of greater depth to the picture, 
always provided the optical system be re-aligned to fit the 
new lens diameter, as per instructions given. 

WARNING. — Diagram Fig. 58 must always be drawn to fit 
the diameter of the projection lens you are now using, regard- 
less of whether the lens is as large as indicated by Fig. 58 A 
and table No. 8 or not. 

When we have finally determined what focal length lenses 
are required, we go a step further. A 6.5 collector lens will 
reduce waste between the two lenses of the condenser to a 
minimum, hence we install a 6.5 collector lens in all cases, 
adding the difference to the focal length of the converging 
lens. Thus : Suppose we find that two 8.5 focal length lenses 
are necessary. We establish a 6.5 collector lens, which is 
(8.5 — 6.5) 2 inches, less than the focal length required. We 
therefore add 2 inches to the focal length of the converging 
lens, which makes it a (8.5 + 2) 10.5 lens, so that the actual 
requirement is a 6.5 collector and a 10.5 converging lens, 
spaced not to exceed l/16th of an inch apart. The E. F. of 
this combination will be the same as the E. F. of two 8.5 
inch lenses. 

FOR SIXTY AMPERES OR LESS.— We would advise the 
use of the lens chart, page 193. The use of the diagram 
Fig. 58 is not advised for anything covered by the lens chart, 
since the charts are accurately worked out, whereas there is 
always the possibility of error when using the diagram, es- 
pecially by those unaccustomed to making drawings and 
working to fine measurements. 

The use of the diagram Fig. 58 sounds quite complicated, 
but it is really very simple indeed, if followed througn step 
by step. 

WARNING. — Projectionists and exhibitors are warned that 
unless the various elements of the projector optical system 
be intelligently selected and carefully and intelligently ad- 


justed with relation to each other the best results will not 
be had on the screen. There will not only be waste of light 
but unevenness of illumination of the screen, with consequent 
loss of depth, or stereoscopic effect. 

CONDENSER DIAMETER. Note— The following is in- 
tended to apply ONLY to the plano-convex condenser. The 
Cinephor parabolic condenser will be dealt with elsewhere. 
See page 212a. The diameter of the condenser has an imme- 
diate and a very important bearing both upon the amount 
of light available to the screen from a given amperage, and 
upon its quality. 

The large diameter (4.5 inch) condenser in almost universal 
use in the United States and Canada has very much greater 
light collecting power than has the small diameter (3 and 3.5 
inches) condenser used in some other countries. As com- 
pared with a 3-inch lens the 4.5 inch diameter condenser has 
just about 2 times the area. At first glance we would be in- 
clined to say the smaller lens would, nevertheless, have the 
greater proportional collecting power because the strongest 
light flux passes through the center of the lens, but in this 
we would be either entirely in error, or partially so, because 
while it is true the strongest light flux strikes at approxi- 
mately the center of the collector lens, if the crater angle be 
correct, still the greater area of the outer zones of the col- 
lector lens much more than compensate for this, which makes 
the collecting power of the outlying zones of high value, or 
would ma-e them of high value were the quality of the light 
the same for all zones. 

The fact is, however, that while the light source delivers 
light of equal quality to all zones of the collector lens, the 
heavy angle at which it reaches the outer zones sets up a 
tendency to chromatic aberration. 

The color thus produced are carried down into the center 
of the light beam at the spot by spherical aberration, thus re- 
ducing the whiteness and brilliancy of the entire spot. 

There is, therefore, something more than a possibility that 
the smaller diameter condenser may after all be as efficient, 
if not even more so than the standard 4.5 diameter now in 
use, especially in view of the fact that the crater may be 
established nearer to the smaller, thinner lens. 

Then, too, there is another phase of the subject to be con- 
sidered, viz., the condition which in itself prevents the use of 
the full diameter of a standard 4.5 inch condenser. Fig. 59, A 
is the drawn-to-scale diagrammatic representation of an 
ordinary plano-convex 4.5 inch diameter condenser, having 
an actual 4.25 inch free opening, its front surface located 16 
inches from the aperture. The projection lens has a working 



Figure 5' 

distance of 6 inches and 
a free opening of 2.25 

Examining diagram A, 
Fig. 59, we readily see 
that, except for light 
carried down into the 
effective beam by spher- 
ical aberration, no light 
from the condenser zone 
labeled "lost light" can 
possibly enter the pro- 
jection lens under the 
conditions shown. It is 
therefore a fact that un- 
der this condition the 
diameter of the con- 
denser may be reduced 
without loss of screen 
illumination, provided the 
reduction of diameter be 
not carried beyond lines 
X-Y of the upper dia- 
gram. In fact the re- 
duction will serve a good 
purpose, as before set 
forth, in making the re- 
maining beam more 
white and brilliant at the 
film plane. 

In diagram B, Fig. 59, 
we see the condense* 
diameter thus reduced 
by means of a diaphragm 
which eliminates the 
outer zones of the con- 
denser, reducing it to a 
diameter which enables 
the projection lens to re- 
ceive its entire beam. In 
the condition set forth 
by diagram A, Fig. 59, 
the light from the outer 
condenser zones is worse 


than wasted, for the above reasons, and for the further 
reason that, as has already been set forth (see page 182) 
this condition causes unevenness of illumination on the screen, 
with accompanying loss of picture depth. 

This whole matter forms an interesting field for future 
study and investigation. It is one in which the projectionist 
using heavy amperage is especially interested, but the local 
condition must always govern, and whether it is best to reduce 
the condenser opening, increase the projection lens diameter 
or utilize the entire ray by applying diagram 58, is something 
the individual projectionist must decide for himself. 

Everything considered we incline to the opinion that the 
better procedure would be to equip projectors with a standard 
4.5 inch diameter condenser, and to add an iris diaphragm 
by means of which the projectionist may reduce the con- 
verging lens opening to fit the local condition. 

made by those not well grounded in projection optics to use 
a large diameter uncorrected condenser. An uncorrected lens 
even as large as 6.5 inches has been experimented with. It is, 
of course, quite entirely possible to use such a lens, but in 
order to use it efficiently the condenser would have to be a 
great distance away from the aperture. This would mean a 
long focal length condenser, and the light source would have 
to be located a considerable distance from the face of the 
collector lens. In fact, while we have made no actual experi- 
ments to determine the matter, we believe that in using the 
large diameter condenser the necessarily greater distance of 
the light source from the face of the collector lens would 
just about be compensated for by the increased diameter of 
the lens. 

On the other hand, a lens of this diameter would be com- 
paratively thick, and much more costly. The necessarily 
greater distance employed between a lens of this diameter 
and the aperture would make the projector an unwieldy piece 
of apparatus, which could not be housed in the average pro- 
jection room as now constructed. Everything considered it 
is quite safe to say that future procedure will in all human 
probability tend toward a reduction in condenser diameter 
rather than an increase. 

AERIAL IMAGE— WHAT IT IS.— The aerial image of the 
condenser is an image of the front surface of the converging 
lens which is present at a certain point in the light beam. 


It is made visible if a suitable screen be held in the light 
beam at the proper distance from the projection lens. 

ROTATING SHUTTER.— The rotating shutter (see page 
644) can hardly be termed anything else than an integral 
part of the optical system of the projector, since it works 
in direct conjunction therewith. Its presence, or rather the 
necessity for its presence, operates to prevent the use of 
projection lenses of very large diameters. It is not our pur- 
pose at this particular place to go deeply into the treatment 
of the rotating shutter in all its various phases (see page 
644), but merely to give you the effect of distance of the 
shutter from the projection lens. 

The shape of the light ray in front of the projection lens 
varies with the local condition. In the case of a very short 
focal length projection lens the beam may emerge from the 
projection lens divergent, the amount of divergence de- 

Figure 60. 

pending upon the focal length of the lens, or put in another 
way, upon the projection distance and the width of the 
picture. With very long focal length lenses the aerial image 
may be a considerable distance in front of the projection lens, 
and between the lens and the image the ray may be either 
parallel or even slightly converging. The correct position 
for the shutter is at the aerial image, if it is practical to 
place it there. The position of the aerial image may be de- 
termined in a number of ways. It may be found by holding 
a bit of dark colored paper in the light ray, moving the 
paper back and forth until a sharp image of the condenser 
is obtained. When the image is at its sharpest the paper is at 
the position of the aerial image. Another method is to place 
a circle of sheet metal or cardboard, in the center of which 
a hole % of an inch in diameter has been drilled, against the 
face of the converging lens. Then, first having opened the 


projector gate and turned the rotating shutter until the lens 
is open, project the light through and blow smoke in the 
beam in front of the projection lens. 

A point will be found where the light beam will narrow 
down as per Fig. 60, and the narrowed point of the ray will 
be the plane of the aerial image. If a cardboard disk is used 
against the condenser you must work fast when making this 
test, else the cardboard may get hot, smoke and dirty your lens. 

Still another method, and perhaps the quickest and best one, 
is to project the white light to the screen and then pass a 
piece of metal or cardboard downward through the light 
beam in front of the projection lens. If the obstruction is 
between the lens and the aerial image the shadow will show 
first at the bottom of the screen, but when the obstruction 
is made near the position of the aerial image a shadow will 
appear from both directions, and when these shadows meet 
exactly in the center of the screen you are obstructing the 
beam at precisely the plane of the aerial image. 

And now, frankly, here is a point on which we are not yet 
altogether certain. If the beam be parallel from the lens 
to the aerial image there are those who hold that the rotating 
shutter may be placed at any point between the lens and the 
image with equally good results. We, however, hold that the 
point of the aerial image is better, because at that point a 
dissolving effect is had, and we believe that this dissolving 
effect will enable the projectionist to trim down his master 
blade somewhat more than can be done if the shutter be placed 
at any other point, even though the diameter of the ray at the 
other point be equal with the diameter of the ray at the aerial 
image. We do not make this statement of absolutely known 
fact, but only as setting forth our opinion in the matter. 

If the light beam be parallel (of one diameter) from the lens 
to the aerial image, it is best to place the shutter at the aerial 
image plane. This is because of the fact that if the shutter 
be there, then any light which reaches the screen while the 
shutter is in motion (meaning that if the master blade be not 
wide enough to entirely close the lens before the film starts 
to move, or opens the lens partially before the film entirely 
comes to rest) will be distributed over the entire screen, hence 
would have less tendency to injure definition than if the shutter 
were elsewhere, in which case it would only be distributed 
over a portion of the screen, hence would be brighter and 
much more likely to injure definition. 

Note : Under some conditions the aerial image will have 
greater diameter than the beam at a point nearer the lens, 
When this is the case it probably will not be advisable to place 
the shutter at the aerial image. First find out what your con- 
ditions are and then use judgment and common sense. 

The only benefit in locating the revolving shutter at the 
narrowest possible point of the light ray, usually the aerial 
image, is that at that point it is possible to reduce the width 



of the master blade of the revolving shutter to its lowest 
possible value, thus allowing the passage of the greatest pos- 
sible percentage of the light, and also probably securing a 
better optically balanced revolving shutter. 

SHUTTER. — Any increase in working diameter of the pro- 
jection lens calls for an immediate increase in the width of 
the master blade of the rotating shutter. This is true even 
though the smaller lens accommodated the entire light ray, 
unless the larger lens be diaphragmed down, since that part 
of the diameter of the lens which is not working will pro- 
ject a halo of reflected light, and that light is often suffi- 
ciently strong to cause faint travel ghost unless it be cut off 
by the rotating shutter. That portion of the edge of the 
rotating shutter master blade which intercepts light ray 
runs at a certain given speed at the point it intercepts the 
light beam, the speed being, of course, dependent upon the 
distance of the center of the light beam from the center of 
the shutter shaft. If the light beam be 1.5 inches in diameter, 
it follows that it will take the edge of the shutter blade 
traveling at a given speed a long time to cut through the 
beam and obliterate the picture from the screen than it would 






Figure 61. 

if the beam were only one inch in diameter, and if the beam 
be 2.5 inches in diameter there is a decided difference in the 
time requirement as against the beam 1.5 inches in diameter. 
It must be remembered that in any event the revolving 
shutter eliminates approximately 50 per cent, of the total 
light passing through the aperture. This does not mean that 



50 per cent, is eliminated by the master blade, but by the 
master blade and the other necessary shutter blades. It is 
highly desirable to get all the light through to the screen 
that it is possible to get without setting up travel ghost or 
other injurious effects, therefore the projectionist should study 

Figure 62. 

his local condition carefully and ascertain any possibility 
there may be for improvement through altering the distance 
of the revolving shutter from the projection lens. We can- 
not well do more than give you an outline of the problems 
involved, because as we said in the first place, local con- 
ditions vary so much that the projectionist himself must be 
depended upon to determine what is best in his own indi- 
vidual case. However, unless he himself thoroughly under- 
stands the problems involved he cannot do this and will in 
consequence probably have his revolving shutter working 
inefficiently as regards the amount of light delivered to the 
screen; also he very possibly will have a worse condition as 
regards flicker than he need have. 

may be criticised for attempting to teach the projectionist 
such things as this. If so we have no apology to offer. The 
projectionist is forced to use directed light — rays directed by 
lenses — in his profession. The steam engineer who does not 
understand the action of steam, and who could not trace its 
action under any given set of ordinary circumstances, would 
be immediately set down as an ignoramus. He would be 



ridiculed by his brother engineers, and rightly too. He uses 
steam. He should understand it. The projectionist uses 
lenses and refracted light. He should understand them. 

More and more we are becoming convinced that there is 
no more real mystery about light action than there is about 
steam. Opticians like to make a mystery of their profession. 
It is remunerative for them to do so. They insist that even 
the most simple problem in light action be worked out by a 
very wonderful and intricate process. 

John Griffith has evolved a method of tracing light action 
through lenses which we would ask that you examine with 
an open mind, remembering that, as Griffith aptly says, while 
it may not be scientifically correct, it nevertheless IS PRAC- 
TICAL; also, it is a thing any of you may understand and 
apply in practice. 

GRIFFITH'S PLAN.— By experience we have found that 
approximately correct results may be obtained when a set 
of simple lenses, such as a condenser combination, is under 

Figure 63. 

consideration, by considering the center of power of the lens, 
or the combination, as being a single plane. 

Let us, for example, select a bi-convex lens. In scientifically 
correct procedure there are two planes from which meas- 
urements should be made, but we may, nevertheless, for the 
sake of convenience, consider the center of the lens as a 
single plane from which a single measurement may be made. 
True, there will be some error incident to this method of 



procedure, but it will be negligible as compared to the error 
due to spherical aberration. 

The same plan applies to a plano-convex condenser com- 
bination. There are in fact two distinct planes from which 
measurement should be made, but a single plane located at a 

Figure 64. 
position between the two real planes which will vary with 
the radius of the convex surfaces of the lenses, will serve. 
This plane will be nearest the surface having shortest radius. 

Very unscientific, yes, but it is practical, while the scientific 
method of two measurements is not practical, unless spherical 
aberration be taken into account, a thing we have yet to 
see any of the scientific men even attempt to do. 

There are 2 rules in optics to which attention is directed, 
viz.: (A) When the object and image are both located at a 
distance from the lens plane equal to twice its E. F. they 
will be equal in size. (B) The relative size of object and 
image are in proportion to their respective distances from 
the lens plane. 

It may be assumed that most opticians are familiar with 
the wording of these rules, but how many of them have 
reasoned out what the rules really mean? How many of 
them realize that rule A means that, no matter where the 
object and image may actually be, there are always two 
planes, located a distance equal to 2 times the E. F. of 
the combination (or the focal length if it be a single lens) 
on either side of the lens plane, and that these planes are 
exact duplicates of each other, except that one is inverted. 
Yet if rule A is correct, this must be true. By this we mean 
that if a ray passes through or reaches a certain point in 
the image plane (see Fig. 62) after refraction, its incident 


direction must be along a line which will pass through the 
point in the object and a point in the object plane (see Fig. 
61) the same distance from the optical axis that the re- 
fracted line passed through the image plane, although the ray 
may actually have its origin on either side of the object plane 
and at any practical distance from the object plane. 

It is thus seen that if we desire to trace the path of any 
particular ray in a diagrammatic way, as illustrated in Fig. 
62, we would first ascertain the E. F. of the combination, or 
the focal length if it be a single lens, and would establish 
lines A-B, Fig. 62, distant from the lens plane by twice the 
E. F. or focal length of the lens combination. We would 
then draw a line through a point of the source to some 
point of the lens plane and would continue the line to the 
object plane. If we then draw broken line D, Fig. 62, 
joining points E and F, and continue it through the image 
plane line, we have but to draw a line from the point where 
line G joins the lens plane, to and through the point where 
broken line D joins the image plane, to have the direction 
of the refracted ray, which will continue in that exact 
direction until it is intercepted. 

Fig. 61 shows how to lay out the diagram, both object and 
image planes being twice the E. F. of the lens from the lens 
plane, and parallel thereto. Fig. 62 shows how a single ray 
is refracted. 

Fig 63 shows how a bundle of rays are refracted, and Fig. 
64 illustrates the method of finding the image of any point 
of the object. 

First draw line 1, Fig. 64, through a point in the object, 
and continue it until it joins the object and lens planes. 
Having thus established the point in the object plane, we 
next draw a line from it to the image plane, so that it passes 
through the optical axis line at the lens plane. This is 
line No. 2 in Fig. 64, and gives us the point of the image 
plane through which the refracted ray, 1-R, Fig. 64, will 
pass, whereupon a line, No. 3, Fig. 64, drawn through the 
object point and the lens plane at the optic axis, and on to 
the point where it meets refracted ray 1-R will indicate the 
image point. The terms Object and Image planes are used 
merely to identify the planes as shown in Fig. 61. And it 
should be distinctly understood that the object and image 
plane herein referred to have nothing to do with the position 
of the actual object and image. 

It will apply to either single lenses or simple combinations. 




The Bausch & Lomb Optical Company has placed at the disposal of 
the industry a condenser which seems to be of sufficient importance to 
justify a description and the giving of such instructions as are available 
in the Bluebook immediately. It is put forth under the trade name 
of the "Cinephor Condenser" and consists of a piano convex collector 
lens and a converging lens ground to parabolic form. The parabolic lens 
corrects spherical aberration — see page 129. Both lenses are made from 
a special highly transparent and heat-resisting optical glass designed 
to reduce light absorption and breakage to a minimum. 

The Cinephor is still too new (July, 1923), to have enjoyed a very wide 
test in actual projection practice, but such reports as have reached 
us from projectionists have been favorable. The author of this book 
and John Griffiths have both examined carefully into the theoretical merits 
of the Cinephor and we both have concluded that it represents a decided 
advance over anything heretofore presented in the way of a condenser 
for the motion picture projector. 

Examine the various figures on pages 181 to 193, inclusive, and the 
text matter therewith, and you will understand the tremendous dis- 
advantages the old style condenser labors under. Except under very 
favorable conditions, never present with high amperage and long focal 
length projection lenses, the piano convex or the meniscus bi-convex 
condenser sets up a condition under which a large diameter projection 
lens must be used, with consequent reduction of depth of focus, an 
increased width of rotating shutter blade and a probable unevenness 
of screen illumination through inability of the projection lens to admit 
all the light beam, as per figure 47, page 182. 

The Cinephor avoids all this, either completely or in large measure, 
and certainly a thing which does this without setting up other difficulties, 
is worthy of very serious consideration. 

FIG. 10 A 

Figure 64-X 

Showing difference in action of beam, as a whole, from Cinephor 
(below) and uncorrected (above) condenser. In other words, 
when the Cinephor and the straight plano-convex condenser 
is used. 


DIRECTIONS: The following directions for use of the Cinephor have 
been prepared by the engineers of the Bausch & Lomb Company at 
our request. We present them as the best data now available, with 
notation that it is possible that after the Cinephor has been out for a 
longer time they may— or may not— be subject to modification. 


There should not be to exceed one- fourth (y) of an inch between the 
surfaces of the two lenses. THE ACCOMPANYING TABLE IS BASED 
INCH CRATER DIAMETER, and since diameter and heat will vary with 
varying amperage, the table is subject to modification in, accordance 
therewith. In other words, the table will only be found to be exactly 
correct when the amperage is such that a crater of ty$ inch HORIZONTAL 
diameter is used. 

HIGH INTENSITY. If a high intensity arc be used it will seldom if 
ever be practicable to use a collector lens of less than 7y inch focal 
length, because of the intense heat of the high intensity arc. When a 
high amperage ordinary arc is used it may also be found necessary by 
reason of the heat to use a collector lens of longer focal length than is 
indicated in the table. In this we must depend upon the common sense of 
the projectionist. He must remember that while excessive lens breakage 
cannot be tolerated, still the use of a condenser of such focal length as 
will locate the crater an unnecessary distance from the face of the 
collector lens will set up a heavy and unnecessary waste of light. 

Table No. 8 A gives the focal length of the piano convex collector lenses 
which will work best with the parabolic converging lens when working 
with projection lenses of various focal lengths from four to eight and 
one-half inches E. F. with both a ten and a twelve-inch distance between 
face of converging lens and projector aperture. 

Table 8 A 


Face of Converging 

Lens Distance 

Face of Convergin 

g Lens 


Aperture 12 inches 


Aperture 10 inches 

E. F. of 


Focal Length 


Focus of 


Crater to 


Crater to 







4 inch 










4/ 2 

3*/ 4 




4V 4 















5/ 2 







7/ 2 





7/ 2 






















s/ 2 


















UNDER the broad term "projection/' many things are 
grouped. After the work of the actor, the director, 
the cameraman and all those other various ones hav- 
ing to do with the production of the photoplay is finished, 
the product only has commercial value when it is finally 
presented to the public on the screen. 

Presentation of the photoplay and projection of the photo- 
play are terms with entirely different meanings. Projection 
includes only those actual things necessary to the placing 
of the picture on the screen. Presentation of the photoplay 
includes its projection and all those various things which go 
to (a) make the theatre patron comfortable ; (b) to properly 
synchronize appropriate music with the picture; (c) to make 
the surroundings pleasing; (d) to make proper lighting of the 
auditorium, and in fact all those various things which gc to 
make the finished whole pleasing to the audience. 

It is only with projection that we are, however, concerned. 
We believe there are very few who realize to what an extent 
the whole motion picture industry rests on the final act of 
projection. It may be stated as an incontrovertible fact that 
the success or failure of any photoplay, insofar as concerns 
any individual audience, will to a very considerable extent 
rest upon the excellence of its projection. We do not believe 
any person conversant with the facts will dispute the state- 
ment that inferior projection will mar the production— make 
it less pleasing to the audience. Nor do we believe anyone 
will dispute the proposition that the more pleasing the 
screen results, entirely aside from any merit it may have as 
a play, or the "pulling power" of the artist therein, the 
greater will be the patronage of the theatre. 

On the other hand certainly no one will even question the 
statement that a dim, "fuzzy," unsteady projection of a 
photoplay will be far less pleasing to an audience than will 
a perfect projection. 

Most of the readers of this book have visited theatres 
where the projection is very good indeed, and have also 
visited theatres where the projection is very poor indeed. 
You all know what the relative effect of those two conditions 


manager wants the result, he is not willing to do the pre- 
liminary work necessary to enable the show to be run to 
schedule without injury. In Fig. 64A the effect of certain 
things is visualized. In this diagram, which is the work of 
L. Davis, projectionist, New York City, we see the effect of 
running to schedule with varying footage. For instance, 
having a seven-reel show, lasting one hour and fifty minutes, 
it will be necessary to run at an average of 15.7 minutes to 
the reel, whereas if the number of reels be increased to 
9 then the speed would be an average of 12.2 minutes to the 
reel. And right there comes the abuse of the schedule. The 
manager has a fixed schedule of, let us assume, 2 hours. To- 
day he has a show of 8 reels, which will be an average of 
15 minutes to the reel, provided the whole time be given over 
to projection. But tomorrow an extra scenic is shoved in. 
The schedule remains fixed, but the time is, by this pro- 
cedure, automatically speeded up to 13.3 minutes per reel. 

Where a fixed schedule is employed, there is and can be 
but one proper procedure, viz: the show must first be pro- 
jected at proper speed, the required time for such projection 
noted, and enough taken from or added to the program to 
enable the projecting of the show at proper speed in the limits 
of the schedule. This is one of the big vlaues of tableau, 
orchestras and prologues. They can be utilized to fit the 
show to the schedule, which cannot always be done where 
the show is entirely photoplay. 

OVER-SPEEDING.— One of the cardinal sins of the the- 
atre is over-speeding projection. Former President Wilson 
once said, within hearing of the author: "I have often seen 
myself in motion pictures, and the sight has made me very 
sad. I have wondered if I really do walk like an animated 
jumping jack, or move around with such extreme rapidity as 
I appear to." 

President Wilson did not know what caused it, but you 
and I do. It was over-speeding of projection. Over-speeding 
(a) increases the speed of action of all moving things; (b) 
sets up heavy strain on the entire projector mechanism and 
on the sprocket holes of the films. Over-speeding is repre- 
hensible from any and every viewpoint. It is practiced by 
managers and projectionists who have no respect for the 
property entrusted to their care and no adequate conception 
of the business of exhibiting motion pictures or of their duty 
towards their patrons. 

Over-speeding projection produces a ridiculous travesty on 


the original, the amount of which will depend upon the rate 
of over-speeding. There are managers and projectionists 
who talk learnedly about a/ reel requiring "15 minutes" or 
"18 minutes," in blissful ignorance of the fact that their 
words convict them of having slight adequate knowledge of 
artistic projection. 

As a matter of fact in any given production the speed of 
projection is likely to vary with individual scenes, and, as 
a whole, with individual reels. 

The correct speed of projection is the speed at which each 
individual scene was taken, WHICH SPEED MAY, AND 

A cameraman out on location encounters bad light condi- 
tions. He slows down camera speed to the limit, in order to 
get all the light he can. The next scene to this was per- 
haps taken in a studio, with perfect lighting conditions and 
at maximum speed. One may have been taken at 60 and 
the other at 70. It requires no extraordinary brain power to 
understand that if the projector pounds along through both 
scenes at 60, one scene will be correctly portrayed and the 
other will be entirely too slow, or if the projector runs at 
70 one will be correct and the other entirely too fast. On 
the other hand, if the projector runs both at 65 then both 
will be wrong. 

One of the highest functions of projection is to watch the 
screen and regulate the speed of projection to synchronize 
with the speed of taking. 

Of course if cameramen always took scenes at one speed, 
all that would be necessary to perfect speed of projection 
would be to set the orojector speed at camera speed, but 
the fact of the matter" is THERE IS NO SUCH THING AS 
A SET CAMERA SPEED. Camera speed varies all the way 
from 60 to as much as 85. 

The projectionist who regards the finer details of pro- 
jection as not of sufficient importance to justify him in giving 
them attention is not, and in our opinion never will be, a high 
class man. Nor is the projectionist excused by the fact that 
many managers impose schedule limitations which render 
high class work impossible, or in other ways hamper his 

Many managers do impose conditions which render high 
class work impossible, but the fact remains that the man 
who persistently and consistently bends every energy to im- 
prove his projection in every possible way is, in the end, 


bound to win. It may and probably will require considerable 
time; it may be discouraging, but success will finally come, 
and with it, at least in some degree, financial reward. 

The manager who employs a high class projectionist, pays 
him an adequate salary, provides him with good working 
conditions, tools and supplies, and insists on high class pro- 
jection, may not immediately see the benefit. The fact re- 
mains, however, that in due course of time the public will 
recognize the fact that in a certain theatre they are sure to 
see a perfect screen result and other things being equal the 
effect of this will be made visible at the box office. 

We might expend pages in setting forth interesting and 
valuable matters pertaining to the broad subject of projection, 
but inasmuch as space is limited, and the whole book is 
really devoted to that subject, we will end by saying: 

Over-speeding the projector is an outrage on the public; 
an outrage on the producer; an outrage on the projector 
manufacturer; an outrage on the film exchange and an out- 
rage on the projectionist himself. There is and can be no 
excuse for it — absolutely none whatever. If the house is full 
and a crowd waiting to gain entrance it would be far better 
to eliminate one reel of the program than to butcher the 
whole performance. 



The Screen 

(Important: See also "Characteristics of screen surfaces" 
Page 483, Volume II.) 

THE sole and only function of the screen is that of re- 
flecting "picture light." The eye sees the picture precisely 
for the same reason it sees any other visible object. 
Exactly as light rays are reflected from any visible object to 
the eye, so in projection light rays are reflected by the screen 
surface to the eye. The picture appears plainer, sharper and 
in every way better if the picture light be abundant — the 
screen brilliantly illuminated — than if it be dull, or if light 
other than picture light be also reflected. However, reflected 
light should also be of the proper color or tone value so that 
it will be soft and restful to the eye. An over brilliant screen 
is worse on the eye than a dimly lighted one. 

The term "picture light," as used here, may be under- 
stood as meaning light projected to the screen from or by 
the projection lens; "other light" is light reaching the screen 
from sources other than the projection lens, which latter, 
being undirected by the lens, shines indiscriminately upon 
both whites and shades of the picture, thus dulling the con- 
trast and causing the blacks to appear gray. 

There is a great difference in screen surfaces and the re- 
sults had from different ones with a given intensity of 
picture light. This is likely to be especially true when the 
screen be viewed from various parts of the auditorium, but 
until quite recently there has been no dependable, authorita- 
tive data setting forth the characteristics of various sur- 
faces used for projection. The author had in mind the 
making of certain measurements and tests for the fourth 
edition of the handbook, and had in fact already done con- 
siderable preliminary work thereon when the Society of Motion 
Picture Engineers made available tests of screen surfaces 
which made that work on our part unnecessary. We used 
this data in the fourth edition, but it was far out of date 
when this present edition was compiled, hence we arranged 
for the compilation of new data, which we shall present 
further along. Such data has enormous value in that it 
enables exhibitors and projectionists to work intelligently in 
the matter of selecting screen surfaces for auditoriums of 
various depths and widths. 

A FALSITY.— In the past many exhibitors and projection- 
ists have based their judgment of the efficiency of various 


screen surfaces on looking at the performance of screens in 
different theatres. 

This is an utterly unreliable test, because it almost never 
happens that there are two screens in neighboring theatres 
where the various factors which may affect the result are 
of equal value, and the working conditions precisely alike. 

In any two theatres the brilliancy of projection light may, 
and in all human probability will, have different value, 
because of differences (a) in amperage at the arc, (b) in 
crater angle, (c) in the carbons themselves, (d) in the con- 
densers or their spacing or discoloring, (e) in the general 
adjustment of the optical train, (f) in the projection lens 
diameter or working distance, and (g) in the revolving 

The result may also be very much altered by the decora- 
tions of the theatre, by its lighting, by the number and ar- 
rangement and power of the orchestra lights, by the screen 
surroundings, the screen border, the shape and height of 
the auditorium, size of the picture, angle of projection, etc., 
etc., through a long list. 

In fact the things affecting apparent screen brilliancy in 
any given theatre are so very many that the judging of 
relative screen values by observing the picture in various 
theatres is an utterly futile endeavor. 

It is even impractical to judge closely of values by sub- 
stituting one screen surface for another while a picture is 
running, because of possible differences in light values. Sup- 
pose we run half a picture on one screen and then drop an- 
other down to receive it, but the crater angle has, unknown 
to even the projectionist, changed, or the supply voltage has 
dropped, thus altering the light brilliancy considerably. 

The only way such a test can be made with any assurance 
of reliable results is to cover half the screen with the sur- 
face it is desired to test, and then project a picture, observing 
results from all parts of the theatre. 

This is a test which is in every way fair; also it is not a 
difficult one to make, but exhibitors and projectionists should 
remember that it is not to be expected that the surface of a 
screen which has been in use for a considerable time can 
enter into successful competition with a new surface. 

We would most emphatically warn exhibitors, projection- 
ists and theatre managers of the danger of judging hastily 
as between various screen surfaces. We would also caution 


exhibitors, managers and projectionists against the too ready 
acceptance of statements made by screen salesmen. 

Those gentlemen are employed to sell goods. Their job 
depends upon their ability to do it, and we have known of 
cases where they did not confine their statements as to the 
merit of their own goods vs. the screens made by others to 
quite the exact facts. 

We have known of many cases where exhibitors have paid 
substantial sums of money for new screens which were in 
fact inferior for use in their theatre to the screen the new 
one displaced. In such cases the exhibitor might better have 
taken his money out and scattered it in the street. 

Very careful consideration should be given to the nature of 
the surface it is proposed to install. It must be understood 
that detail in the shadows is lost when viewing a highly 
illuminated picture on screens of a specular nature. This is 
because of the fact that the highlights are brilliant and harsh, 
the effect of which is to cause the pupil of the eye to contract, 
in an endeavor to shut out the glare. This should be self- 
evident to any one who understands the action of the human 
eye, and a contracted state of the pupil of the eye does not 
help us to see the finer detail in the shadows. 

Under this condition the picture w r ill lack detail. There is 
the impression of bright lights and deep shadows, but little 
or no intermediate shadows or "halftones." That this is true 
may be demonstrated by viewing two pictures projected side 
by side, one on a highly reflective surface and one on a sur- 
face of less brilliancy and higher powers of diffusion. 

The matter may also be roughly tested, as follows. When 
an interior scene is projected to a brilliant screen surface, 
such as, for example, a dining room having dark walls and 
furniture, with a white table cloth on the table, pay especial 
attention to the matter of visible detail in the shadows. Hav- 
ing done this for a moment or two, hold your programme, or 
other opaque object some distance in front of you, in such a 
way that the white table cloth will be cut out of the range 
of vision, but the darker portions of the room will still be 
within the visible field. In a moment, after the sharp glare of 
the table cloth has been eliminated and the pupil of the eye 
has had time to expand to meet the new condition, you will 
find that the shadowy parts of the room, which were flat and 
dark — without detail — have rounded out, and are full of visible 
detail. Remove the object so that the cloth is again visible, 
and watch the rest of the scene become relatively dark and 
without detail. 


The reason for this is that the eye, in the very nature of 
things, cannot accommodate itself to high brilliancy and rela- 
tive darkness at one and the same time, without the latter 
suffering. The eye accommodates itself to its own comfort 
with regard to the brightest portion of the theatre, and dis- 
regards the darker portions. 

It is therefore important that, except where auditorium 
depth demands high illumination, regardless of the effect on 
detail, the screen surface be of such nature that the high- 
lights will be at least to some extent subdued, particularly 
as regards the blues and violets, in order that the eye may 
accommodate itself to a more or less neutral position with 
regards to highlights and shadows. 

It is a part of the duty of projectionists to study screen 
surfaces and be able to advise the theatre management in- 
telligently in such matters. 

We would advise exhibitors to under no circumstances pur- 
chase screens until they have first carefully examined the 
various charts and tables herein contained, and ascertained 
the characteristics of such surfaces, subject only to later, 
amended data published from time to time in the projection 
department of the Moving Picture World. 

If for any reason it is not deemed expedient or advisable to 
follow the foregoing advice, then we would advise against 
the purchase of any screen until the maker or dealer has 
covered half your theatre screen with a sample of the sur- 
face he is selling. 

There is absolutely nothing impractical in the latter. It is 
quite possible for a screen salesman to carry a sample of 
that size. Once such a sample is in place it is very essential 
that the result be viewed from every portion of the house, 
including the balcony, if there be one. 

Don't attempt to judge from a small sample. Unless you can 
determine the characteristics of the surface under considera- 
tion by the tables and charts submitted herewith, oblige the 
salesman to cover half your screen, or else escort him to the 
front exit and bid him a polite but firm good bye. 

As already set forth, the only function of the screen is to 
reflect light. It therefore follows that in order to under- 
stand the results emanating from any given screen surface 
we must first understand a few of the many laws which 
govern light action. 

SPEED OF LIGHT.— Light travels at the terrific speed of 
186,000 miles per second, a speed so tremendous that there is 
no way of controlling it, hence light speed has a fixed value 


221 y A 



which cannot be altered. This item is of no particular in- 
terest to the projectionist, except as a matter of general 

LAR. — There are three kinds of reflection, viz., regular, semi- 
diffuse and diffuse. Regular reflection occurs when light 
strikes a smooth, polished surface, and is not broken up and 
scattered. An example of regular reflection is the ordinary 
mirror, in which we see ourselves because light is reflected 
from the surface of our face to the glass, and by the glass 
is reflected back into our eyes without being scattered or 
diffused. This type of reflection is illustrated in Fig. 65. 






Figure 65. 

In regular reflection angle A is always equal to angle B. Angle A is 
the "angle of incidence"; angle B the "angle of reflection." 

Diffuse reflection occurs when light is reflected to the 
eye from a body which has a roughened unpolished surface, 
which by reason of its roughness, scatters or diffuses the 
light rays. The more evenly the light is scattered in all 
directions the more perfect is the diffusion said to be. 

It must not be understood from this, however, that by 
roughness we necessarily mean a surface which appears 
rough to the eye. The roughness we have reference to is 
termed "peaks and depressions," and these peaks and de- 
pressions may be very minute in size. Smooth plaster is a 



perfect diffusing surface, although it appears smooth to the 
eye and feels smooth to the hand. Tt is diffusing, however, 
because it is not a polished surface, but a surface made up 
entirely of peaks and depressions of sufficient area to break 
up the light. The effect of such surfaces is illustrated in 
Fig. 66. 

"Picture Light" projected upon a screen is reflected back 
from its surface and is scattered in a wide or narrow angle, 
exactly in proportion to the completeness with which the 

surface lacks 
polish and is 
made up of 
peaks and de- 

Light rays 
and the ele- 
ments of the 
screen surface 
which scatter 
them are both 
of an almost 
infinitely small 


Some surfaces 
which have 
been used for 
screens have to 
a certain ex- 
tent both the 
elements of 

polish and of peaks and depressions. Such screens provide 
both regular and diffused reflection, with the result that a 
haze appears before the screen. This haze is the result of 
regular reflection superimposed over or upon the diffused 
reflection. It is a peculiarity of the polished metal surface 
screen, and explains the reason for the failure of many 
homemade metallic projection surfaces. 

VISIBLE ROUGHNESS.— It was for a long while believed 
that screens with surfaces visibly rough had advantage over 
smooth diffusing surfaces, such as plaster hard finish. This 
has been proven to be an error. Always providing the sur- 
. face be of a character to give a high degree of diffusion 
there is no added advantage in visible roughness. 

Figure 66. 


INTERFERING LIGHT.— One of the very worst faults 
encountered in motion picture theatres, insofar as has to 
do with the screen, is interfering light. By this we mean 
any light other than picture light which strikes the surface 
of the screen. Light interference may be caused by (a) 
stray beams from the projection room which strike the wall 
or ceiling of the auditorium, and are by them reflected to 
the screen. These rays usually come from the condenser. 
Their elimination is a very simple matter, (b). Daylight, which 
is a most prolific cause of poor results at matinee per- 
formances. It is nothing short of astounding how little 
attention theatre managers and projectionists pay to the 
thorough excluding of daylight from the auditorium at 
matinee performances. Any daylight which reaches the 
screen, no matter how slight, is highly detrimental to the 
picture. This is such a patent fact that it would seem any 
projectionist or theatre manager would realize and under- 
stand it. (c) General auditorium lighting improperly ar- 
ranged or improperly shaded. This is another point con- 
cerning which many theatre managers display an absolutely 
incomprehensible indifference. We have many, many times 
entered theatres of considerable pretension, which charged 
a top notch admission price, and found the auditorium lights 
literally murdering the picture, or, what is equally bad, found 
unshaded white or brilliant red lights glaring directly into 
the eyes of the audience. Auditorium lighting will be dis- 
cussed under the proper heading, therefore we will not go 
into it further here. 

Exhibitors and projectionists should test their screen for 
stray light occasionally. They may only do this be'st when 
the projector is not working, and the theatre is lighted just 
as it is when the show is on, including the musicians' lights. 
If under this condition the screen does not look the same all 
over, with absolutely no trace of shadows or bright spots, 
then there is something wrong, and the offending light or 
lights causing the shadows or bright spots should receive 
immediate attention. 

After testing the screen thus, with the entrance doors 
closed, open them and see whether bright spots or shadows 
appear on the screen. This latter test should be made in 
the afternoon as well as in the evening if there is a matinee 
performance. If the opening of the entrance doors affects 
the screen, then the steps necessary to protect the screen 


from the light entering through the open doors should he 

Exhibitors who pay perhaps hundreds of dollars a week 
for film service, and who fail to give proper attention to such 
details as these are doing a very foolish thing. Most em- 
phatically a screen which is struck by any light other than 
the picture light itself does not give the best possible result. 

Awnings on the outside of the window will serve very well, 
especially if made of dark cloth, and will not interfere with 

Small town theatres which have windows opening directly out- 
doors may exclude the light effectualy by means of double, 
dark colored window shades, the edges of which run in grooves 
not less than one inch deep. These grooves may easily be 
built by a carpenter, and the plan will be found quite effectual. 
One shade will do, but two are better, since the single shade is 
likely to develop pin holes which will admit light. 

Standing beside the screen, looking toward the auditorium, 
there should be no unshaded light source visible to the eye 
at any point. If there is, then the light from that source is 
reaching the screen and injuring the result. 

Indirect lighting is a most excellent form of auditorium 
illumination, provided it be properly installed, but this is by 
no means always done, the most common fault being the 
installation of fixtures too close to the screen end of the 

Reasonably dark colored, non-reflective wall decorations 
are a great aid in eliminating stray light ; also they are very 
restful to the eye, though by this we do not mean to infer 
that the decorations of a theatre should be sufficiently dark 
colored to be gloomy. 

mental requirement of a screen surface is that it as nearly as 
possible reflect light equally to all seating space in the audi- 
torium, so that the picture appear as brilliantly lighted to 
those patrons seated at a heavy ?ngle to the screen, or to 
those seated in the balcony, as it does to those seated in 
the center of the orchestra floor. If the screen exhibits a 
certain degree of brilliancy to those seated in the center of 
orchestra floor, and a lesser degree as one moves around to 
one side of the house, it is said to have "fadeaway," and just 
in proportion as the screen develops this characteristic it is a 



poor diffusing surface. If the fadeaway be too pronounced 
it is not a good surface to employ for a screen, except in the 
case of a very narrow auditorium, not matter how convinc- 
ingly the screen salesman may talk in the endeavor to con- 
vince you to the contrary. It gives semi-diffusion of the 
light, as per Fig. 67. 

Fadeaway is also bad because if one be seated near the 
screen so that there is a decided difference in the angle of 
vision when viewing different parts of its surface, the part 
of the screen coming under the least angle will be brighter 
than other parts that are under more severe angles. 

SEVEN CLASSES OF SCREENS.— Screen surfaces may 
be divided into seven general classes, viz.: (1) white 
wall, (2) the cloth screen, (3) the kalsomine screen, (4) the 

painted screen, (5) the metallized 
surface screen, (6) the glass or 
mirror screen, and (7) the translu- 
cent screen. 

white plaster 
wall known as 
the "hard fin- 
i s h surface," 
forms an ex- 
cellent surface 
for the projec- 
tion of pic- 
tures, in that 
it has very ex- 
cellent diffu- 
_. .,_ s i o n indeed, 

Figure 67. while at the 

Semi-diffuse reflection, in which the greater part same time its 
of the light is reflected back in the direction whence power of re- 
it came. flection is good 
for a surface of such high power of diffusion. The plaster 
surface may be cleansed by sandpapering it lightly, using 
No. y 2 sand paper, but it can, of course, only be cleaned a 
limited number of times before it will be worn down to the 
brown plaster underneath. 

THE CLOTH SCREEN.— This has as excellent powers of 
diffusion as any surface procurable but its power of reflection 
is very low. It is, therefore, very difficult to get a brilliant 
picture of a theatrical size on cloth. The cloth screen is now 
seldom used except for strictly temporary installation, or by 
traveling exhibitors. If it is proposed to use cloth a good 
grade of bleached muslin of the kind used for bed 
sheeting is best. It is possible to obtain this cloth 108 inches 
in width. A cloth screen should always be tightly stretched. 


so that it will present a smooth, unwrinkled, perfectly flat 

THE PAINTED SCREEN.— There are certain things with 
relation to paints which have been thoroughly established 
by experiment, but which have not as yet been put into form 
for application in practice insofar as applies to the pro- 
jection surface. 

The higher the percentage of pigment contained in a paint 
the higher will be its light-reflecting power. Experiment 
has proven that it is quite possible to produce a paint of 
extraordinary whiteness and very high light reflecting power 
by using heavy bodied oils, cut down by means of volatile 
thinners. It is claimed that by this method flat paints may 
be had which will have from 20 to 25 per cent, higher light 
reflecting power than paints now in general use. 

In ordinary inside house painting it has been found that 
lead and oil paints deteriorate in light reflecting power by 
about 15 per cent, per year, aside from the loss of reflecting 
power due to accumulations of dirt — due entirely to chemical 
changes in the paint itself. 

It has also been found that kalsomines lose in reflection 
power in about the same ratio, but in this case it is en- 
tirely chargeable to absorption of dirt by the porous coat- 
ing, hence the loss will be very much higher in a dusty or 
smoky atmosphere. 

WARNING.— Black should never be used to "whiten" 
screen paint, because the addition of the least bit of black 
acts to seriously lower the reflecting power of the coating. 

The foundation for a painted screen surface may be 
tightly stretched cloth, cement finish, plaster, or anything 
else which presents an even, unbroken surface. There are 
a number of screen paints on the market, but up to date 
(subject to foregoing, which forms basis of interesting study) 
we have seen nothing better than either all zinc white, or 
half zinc white and half of a good grade of white lead, 
mixed to a proper consistency with Y^ boiled linseed oil and 
24 turpentine, adding just enough ultra-marine or cobalt blue 
to give the paint a very light bluish cast while in the pot. 
When spread on the surface the blue tint will disappear and 
the paint appear dead white. Unfortunately, no authoritative 
data as to its reflective power is available, but it must be 
decidedly superior to that of white plaster, and paint is as 
excellent a diffusing surface as any we know of, excepting 
only plaster and cloth. Colors, of course, may be added to 


suit the individual idea, but our own advice is to stick to the 
white paint, without any color of any kind whatsoever. We 
would advise the application of three or four coats of tolerably 
thin paint, rather than a less number of coats of heavy paint. 
The result will be better, and the surface dries very quickly. 
Also it is absolutely imperative that this paint be evenly applied 
and without visible brush marks. 

The painted screen offers an easily and cheaply renewed 
surface, very excellent diffusion, practically an entire lack 
of fadeaway, and the best possible definition of the picture 
from the front rows of seats. Very many high class theatres 
either already have or now are reverting back to the painted 
screen, because of the advantages above enumerated. Every- 
thing considered, it forms one of the most excellent projection 
surfaces yet discovered for theatres when the viewing angle 
exceeds 30 degrees, though a given amount of projected light 
per unit of area will not give as great brilliancy, when the 
screen is viewed from the center of the auditorium, as will be 
had when using either a metallic surface or mirror screen. 

There is, however, a great deal of what we will call self 
deception in the matter of screen paints, or rather in the 
methods of painting a screen. When white paint is applied 
directly to a cloth screen a great deal of light "goes through. ,v 
This light is, of course, lost, and in the endeavor to save it 
some rather weird plans have been evolved. As a rule the idea 
most advanced is that if a coating of semi-opaque color be 
applied to the surface, and sufficient coats thereof be applied 
to make it opaque, and then the white surface be placed over 
this opaque surface, or even if the opaque coatings be applied 
to the back of the finished screen, no light will "go through," 
hence there will be a greater brilliancy. The favorite color 
for the underlying coatings has been blue, or a sort of lead 
color, on the theory that the light, striking the blue and being 
reflected back will whiten the overlying surface. 

The only real value of paint on back of screen is to prevent 
light from rear shining through, when the screen is used as 
a stage. As to light from a backing being reflected 
back, to an extremely limited extent it might possibly be 
true, but while the opaque coatings underneath the white 
will stop the light from going through, it will not and 
cannot add anything appreciable to the brilliancy of the 
screen. The underlying coat stops the light from going 
through for the very simple reason that it absorbs the rays. 
It does not reflect them back, or anywhere else, modified by 
the fact that if the underlying coat be blue it will absorb 
everything but the blue rays, the question then being can the 


blue ray be reflected back through two or three coats of 
white paint, or even appreciably through any minimum 
necessary number of coats. Personally we do not believe 
it is possible this could take place to a sufficient extent to 
have any appreciable effect in whitening the surface, and 
most emphatically it could not add any brilliancy to the 
screen other than what slight effect there might be in 
whitening, which latter effect could be just as well secured 
by adding blue directly to the color. 

FOR AIRDOME SCREENS a paint is necessary. If the 
surface of a theatre screen to be painted be plaster, then it 
should first either be sized with a coating of glue size, or 
painted with a coat of thin shellac. If the airdome screen 
be built of lumber, then we would suggest the installation 
of a painted cloth screen, stretched on a suitable frame, 
over the lumber backing. 

WASHING PAINT SURFACE.— A painted surface may be 
washed, provided it be very carefully done, but the washed 
surface will not have the same brilliancy as a new surface. 

KALSOMINE SURFACE.— A variation of the painted 
screen which we can heartily recommend is the covering of 
either cloth, plaster or cement with one of the patent white 
kalsomines, such as alabastine or muralite, which may be 
had from any dealer in paints, and which may be applied by 
almost anyone after a little practice, though it is, of course, 
always better to have the work done by a competent painter. 
This surface forms a really very excellent screen. In powers 
of diffusion it ranks very close to white plaster. Its reflection 
power and its powers of diffusion will be found set forth in 
table 2, page 492c. It has very high powers of diffusion. The 
surface is easily and cheaply renewed. 

CAUTION. — In kalsomining one must use a good brush, 
and make no attempt to brush out the work smoothly. 
Instead one should swing the brush in every direction. The 
work must be done fast, because if the edge of the work 
dries so that the next "lap" will not work into the first per- 
fectly the joint will show. It is better to have kalsomining 
done by a man experienced in such work. 

Don't imagine that you can coat a cloth or plaster screen 
with kalsomine or paint and use it indefinitely without doing 
anything more to it. We would very strongly recommend 
that where a plaster or cement kalsomine coated screen is 
used, it be washed off and recoated every ninety days. 


It may look clean and bright, but you may be very certain 
it is not. The wall paper or kalsomine on the walls of your 
home may look perfectly clean, but knead some fresh bread 
into a dough, rub the wall paper with it and see what hap- 
pens; perhaps the result will astonish you. Exactly the 
same thing applies to the screen. Kalsomine or paint is 
cheap. Our advice is, USE THEM FREQUENTLY. 

TESTING SCREEN SURFACE.— To test either a kalso- 
mined or painted projection surface place against it a sheet 
of white blotting paper — a desk blotter as to size. If. the 
paper appears whiter than the screen, then the screen is not 
in good condition. 

NEAT CEMENT SURFACE.— "Neat" cement is white in 
color. It has been suggested that it would make an ideal 
screen surface. This suggestion may or may not have value. 
We have never seen such a surface, nor have we any data 
concerning the matter. 

GLUE SIZING. — Before undertaking to coat a cloth screen 
with either paint or kalsomine it must be stretched tightly 
on a frame and thoroughly sized with a solution of glue in 
the proportion of from 1 to 2 pounds of glue to the ordinary 
pail of water. The amount used will depend upon the grade 
of glue. 

to which various compounds containing more or less pow- 
dered aluminum or other powdered metallic substances 
have been applied, have been and still are quite popular, 
though the tendency is to revert back to a surface of less 
contrasting brilliancy, except in theatres having a rather nar- 
row auditorium. 

Metallic surface screens have for their base some kind of 
cloth fabric, to which the metallic substances are applied by 
processes which are held jealously secret by the manu- 
facturers. Their sole advantage lies in the fact that within 
certain angles the projectionist is able to get a very high 
degree of brilliancy per unit of area at considerably less 
expenditure of light, hence of electric energy, than is possible 
with a more perfect diffusing surface. This relatively high 
brilliancy, however, usually only extends over an angle of 
20 to 30 degrees, outside of which angle there is a decided 
fadeaway. See pages 492b to 492f, Vol. II. There is also a 
more or less pronounced tendency to discoloration of the 
surface, especially in damp climates, though this latter fault 


need not worry the purchaser if he secures a proper written 
guarantee against such fault. Such surfaces are, however, very 
contrasty and destructive of detail in the picture, if illuminated 

There is a very decided difference in metallic screen surfaces, 
but the characteristic of the different types may be examined 
in pages, 492c to 492f, Vol. II, and the purchaser may know 
precisely what effect he will get from any given type of screen. 
WARNING.— It is a very difficult matter to apply metals 
(either in powder or paint form) to a screen surface in such 
way as to secure the best possible light diffusion, and the ex- 
hibitor who prefers to use a metallic surface screen should 
by all means purchase the screen from a reliable manufac- 
turer. The manufacturer makes a specialty of preparing such 
surfaces; also he usually applies stretching devices which 
will allow of the screen being properly installed. It is almost 
impossible, and certainly is entirely impractical for the pro- 
jectionist or exhibitor to make a satisfactory home made 
metallic surface screen. 

CHALK SURFACE.— There is possibility for an excellent 
projection surface in common chalk. This surface has to some 
extent been used, and has given excellent results. The surface 
is made by rubbing ordinary white chalk, such as carpenters 
use for their chalk lines, and which may be had cheaply at any 
hardware store, on plaster or any other suitable surface. Even 
school crayons, broken in two and used flatwise, will do. Such 
a surface cost very little in money, but requires considerable 
labor to get it on evenly. The picture stands out on a chalk 
surface with surprising brilliancy. 

Rubbed on a plaster wall a chalk surface may be removed 
for renewal by means of an * ordinary school blackboard 
eraser, and may be renewed by a few cents* worth of chalk, 
plus considerable labor. 

MIRROR SCREENS.— The mirror screen consists of a 
sheet of plate glass, the back of which is coated precisely the 
same as is an ordinary plate glass mirror. Its face Is then 
sand-blasted to a dull finish, which may be made rough or 
smooth, according to the condition under which the screen 
is to work. The light is caught on the ground face and a 
portion of it is reflected back. The rest of it goes through, 
strikes the silver at the rear surface and is reflected back to 
the rough finish. This has the effect of producing a very 
high efficiency and a very brilliant result when the screen is 
viewed from in front; but due, we believe, in some measure 


to the thickness of the glass, there is tendency to out of 
focus when a mirror screen is viewed at a wide angle, because 
of a double reflection — one from the silver back and one from 
the ground front. The satin finish mirror screen is an ideal 
installation for the long, narrow theatre, by reason of the fact 
that the audience will all be seated practically directly in front 
of it, under which condition a very brilliant picture may be had 
with a comparatively low projection light value; also the satin 
finish mirror screen has the peculiarity that the further you 
get away from it, within reason, of course, the more brilliant 
the picture appears. It is, therefore, particularly of value in a 
very deep, narrow house. 

One of the principal objections to the mirror screen is that 
it is costly, difficult to install and subject to some, though 
slight, risk in the item of breakage. Once installed, however, 
barring very improbable accidental breakage, it should require 
no attention, except an occasional washing, for many years. 

TRANSLUCENT SCREEN.— The translucent screen is 
either made of translucent material, such as tracing linen, or 
it is of ground glass. With this type of screen the pro- 
jector may be located on the side of the screen opposite 
from the audience, which will view the picture through the 
screen. The image appears on both sides of the screen, but 
is reversed to the projectionist, to whom all titles and other 
reading matter will read backward. 

When projecting through a translucent screen (called "rear 
projection") the film is placed in the projector with the 
emulsion side toward the screen, instead of toward the light 
as in ordinary projection. 

It is possible to use ordinary cheese cloth or thin muslin 
cloth for a translucent screen, but if this be done the pro- 
jection lens must be sufficiently below the center of the 
screen so that a straight line from the eye of the spectator 
to the lens will not pass through any part of the picture. In 
practice this means that such a screen cannot be used for 
rear projection at all if there is a balcony in the theatre. 
The reason for this is that any spectator who sits in such 
position that the eyes will be in line with any portion of the 
picture and the lens will see the brilliant lens spot through 
the screen. The ground glass, tracing linen and screens of 
similar characteristics break up this bright spot and render it 

If a cloth screen be used the result will be greatly im- 
proved if it is kept wet with water. 


The best screen of all for rear projection is ground glass, 
because it causes but a comparatively slight loss of light; 
also it gives very good diffusion, though it is claimed there 
is advantage in grinding the surface coarse for a wide house 
and fine for a narrow house. 

Tracing linen makes a fairly satisfactory translucent 
screen, its worst feature being that it cannot be had suffi- 
ciently wide, hence the screen must contain a seam which 
cannot be made invisible by any present known process, and 
will show more or less in the picture. 

Rear projection is, however, but very little used. It pre- 
sents advantages where conditions are such that it can be 
properly employed, but in 9 cases out of 10 where it is at- 
tempted the distance of projection is so short that really 
good results cannot possibly be had. In fact rear projection 
usually is employed as more or less of a makeshift. 

Where it is possible to obtain a distance from projector 
to screen which will admit of the use of a projection lens 
of not less than 4 inches E. F., however, rear projection on 
a glass or other high class translucent screen comes pretty 
near being ideal, since the projection room with its noise, 
heat and fire risk may be located entirely away from the 
audience, presumably outside the theatre. 

The question is often asked, can we locate a translucent 
screen at the proscenium line, set the projector at the rear 
of the stage and get a good picture? The answer is an em- 
phatic no! It is never advisable to attempt the projection of 
a picture of a size suitable for theatre work with less than 
50 feet from the lens to screen, and 40 feet may be con- 
sidered as an absolute minimum, understanding, however, 
that real high class results cannot be had at 40 feet unless 
the picture be smaller than is ordinarily suitable for the- 
atrical work. Another very serious objection to this plan is 
that it places the screen altogether too close to the front row 
of seats. 

THE CONCAVE SCREEN.— There is no advantage in the 
installation of a concave screen surface, except possibly in 
cases where the distance of projection is such that a very 
short focal length projection lens must be employed — say a 
lens of less than 3.5 inch E. F. Under such a condition there 
may be some advantage in a concave screen surface, but 
given a normal projection condition any advantage such a 
surface might offer would, in our opinion, be more than offset 
by disadvantages it would present in other directions. In 


our opinion where the condition is such that a projection lens 
of 4.5 inches E. F. or more is required a concave screen pre- 
sents no advantage whatsoever. It is one of those things 
which look very plausible, but which will not stand the cold 
light of critical analysis. 

HEIGHT ABOVE THE FLOOR.— The height of the screen 
above the floor must, of course, to some extent, be governed 
by conditions obtaining in the individual theatre. Where there 
is a stage we believe the general effect will be best if the bot- 
tom of the picture be located quite close to the floor. Other 
things being equal, this, we believe, gives the most nearly life- 
like effect to the picture. 

There is, of course, a distinct advantage in locating the 
picture high up on the wall. In some countries this is the 
almost universal practice, the auditorium floor being left per- 
fectly level. Such location, however, also has very serious 
disadvantages, the principal one of which is the tendency to 
emphasize in the mind the fact that one is looking at a picture, 
and not a real performance; this because of the fact that in 
the home and elsewhere we see the picture hung on the wall, 
and the mind, to some extent, subconsciously connects the 
moving picture which is located high up above the floor with 
the picture on the wall. 

Everything considered we believe that, where it is practical, 
the best general effect will be had by locating the bottom of 
the picture about 6 feet above the auditorium floor where the 
screen is on a wall, and no orchestra is used. If there be an 
orchestra, then it will be better to add perhaps 2 feet to the 
above, in order to, as far as possible, avoid the effect of the 
musicians' lights. 

Be careful about this, however, since it will depend some- 
what upon the pitch of the floor, and if the screen be too high, 
looking at its upper half will be a strain for those in the 
front rows of seats. 

EYE STRAIN. — Many people avoid the photoplay theatre 
either because they have seen motion pictures under circum- 
stances which set up eye strain, or because they fear the 
motion picture will cause injury to their eyes. Eye strain in 
moving picture theatres may be attributed to five main causes, 
viz. : flicker, poor definition, poor illumination, a too large 
picture, and glare spots. 

FLICKER. — It is a fact well understood by most people 
that the pupil of the eye expands and contracts in direct 
proportion to light intensity. The retina of the eye is most 
comfortable and "sees" best at certain given light intensities, 
which vary considerably with the individual. The office of 
certain muscles known as the "muscles of accommodation" 
are to expand or contract the pupil of the eye to let in just 


sufficient light to maintain the value most comfortable to 
the retina. 

When flicker occurs the tendency of the muscles of accom- 
modation is to open the pupil during the period of darkness 
to a point where a greater proportion of light enters when 
the picture is being projected than is "comfortable" to the 
retina. This causes a distinct shock to the retina. Another 
effect is a tendency of the muscles of accommodation to 
follow the rapid alternations of light and darkness, and this 
sets up a terrific strain indeed. 

In this connection let it be clearly understood that when 
flicker occurs it occurs because of some wrong procedure 
somewhere in the process of projection. Flicker can always 
be eliminated by the projectionist if he understands his busi- 
ness, is provided with proper equipment and is unhampered 
by orders from the management which prevent him from 
applying the remedy. 

The screen itself never produces flicker, but where a screen 
of comparatively low efficiency is used, and is later replaced 
by one of the same area, but of higher efficiency, if the same 
amperage be used the tendency to flicker will be increased by 
the added brilliancy of the reflected light. 

The period of darkness remains of the same duration as to 
time, but the light is more brilliant, hence there is added con- 
trast. If the light were reduced until the picture on the new 
screen had no greater brilliancy than the picture on the old 
screen, which may be done by reducing the amperage, it 
would be found that the flicker will be neither more nor less 
than it was before. 

Flicker due to the alternate opening and closing of the lens 
by the revolving shutter of the projector is utterly inex- 
cusable. When it occurs, either the projectionist lacks the 
knowledge necessary to eliminate it; lacks energy to do the 
necessary things to eliminate it, or the speed of projection is 
too slow. 

sharp definition in the picture operates to set up heavy eye 
strain. If you doubt this, have a stenographer do some type- 
writing, making about four carbon copies on ordinary paper. 
The last copy will be "fuzzy." Try to read a page or two of 
that kind of copy and see what happens to your eyes. The 
writing is out of "focus," very much the same as is a picture 
on the screen when it lacks definition. 

Lack of definition may be due to several things. A poor 


lens may cause it. A wrongly adjusted optical system may 
cause it. Oily film may cause it, or the cause may be in- 
herent in the film itself. 

In view of the foregoing, and the importance of prevent- 
ing eye strain in motion pictures now that they have become 
such an enormously popular form of amusement, the theatre 
management should expend some energy and money in 
securing the best possible lenses, the projectionist should 
thoroughly understand the handling of the optical system of 
his projectors, and the projectionist who is careless and 
"sloppy" enough in his work to get oil on the film should be 
promptly discharged. For a manufacturer to send out film 
which cannot be projected in sharp definition on the screen 
is little short of criminal, the crime being against the eye- 
sight of this and future generations. 

tion as an audience becomes deeply interested in the picture 
story, it will make every effort to catch every phase of the 
projected picture. It will closely watch every detail of the 
action, both major and minor, since it sometimes happens 
that upon some slight change of expression, a side glance or 
the wink of an actor's eye, or some comparatively slight 
detail in the action, will hinge very important details of the 
story itself. The audience, therefore, is anxious to miss 
nothing, and if the illumination of the picture be not suffi- 
cient, it is entirely understandable that comparatively heavy 
eye strain may be set up. 

We read the picture story upon the screen almost exactly 
as we read the printed page of a book. If we attempt to 
read a book with a poor light, or when it is shaking or moving 
(jumpy picture) the result is a strain upon the eyes, which may 
be entirely avoided by improving the illumination and holding 
the books still. This is just plain common sense. It is a point 
which even the most obtuse can readily understand. 

Precisely the same thing applies in projection. We improve 
the illumination by projecting more light to the screen through 
the film, or by causing the screen surface to reflect more light, 
and instead of "holding the book still," we prevent the picture 
from "jumping" on the screen. 

Also, be it remembered, that a glary screen surface will "fog" 
the picture, and set up eye strain by injury to the definition. 

screen plays a highly important part in the matter of eye 
strain. If the screen be too large, or, what amounts to the 
same thing, if the front rows of seats be too close to the 
screen, very heavy eye strain will be set up for those occupy- 


ing the front rows of seats. This is for a double reason. In 
the first place, due to the excessive size of the screen or the 
nearness of the seats to it, the eye must travel over a wide 
surface in following the action. It requires no great wisdom 
to understand that this in itself is very hard on the eyes. 
Then, too, if the screen be too large, or if the front rows of 
seats be too close to it, the tendency to eye strain is aug- 
mented, because the picture will not be seen in sharp focus 
from these seats. 

GLARE SPOTS.— One of the most prolific sources of eye 
strain is what is known as "glare spots," which means a rela- 
tively small spot which is highly illuminated as compared to 
its surroundings, and which falls within range of the eye of 
the theatre patron who is looking at the screen. 

Also by over illumination the picture 
highlights may themselves become glare 
spots. See page 221. 

Figure 67 l /2. 

In their pamphlet, "The Motion Picture Theatre, Its Illum- 
ination and the Selection of a Screen" (which we commend to 
the projectionist for addition to his library), the Eastman 
Kodak Company says : 

"No area of the interior of the theatre visible from any seat 
in the audience, except the picture itself, should have an 
apparent brightness of more than 2.5 to 3.0 foot-candles. This 
applies to the walls near a lamp, to the lamp itself if it is not 


concealed, to any diffusing globes or fixtures used, and in 
general to any part of the interior of the theatre. For example, 
a sheet of white paper illuminated by a 25 watt lamp at a 
distance of one foot, has an apparent brightness of about 20 
foot-candles. A sheet of music illuminated in this way, if 
visible from the audience, becomes a glare spot and may cause 
great discomfort. Arrangements should therefore be made 
which, while providing adequate illumination for the musicians, 
will prevent the illuminated sheets from being visible to the 
audience. Lights under a balcony are particularly bad and 
should be used only with a properly designed indirect lighting 
system. Considerable attention should be paid to the character 
and position of exit signs. While it is necessary to make such 
signs very conspicuous, this can be accomplished without making 
them so brilliant as to become disagreeable glare spots." 

Incidentally, this serves to emphasize the importance of sub- 
duing the glare in highlights in the picture itself. 

We agree with every word of the foregoing. Glare spots 
caused by side lights, clocks, wrongly-made exit signs, lights 
on the ceilings of balconies, and music lights, are nothing else 
than a crime against the eyesight of this and future genera- 
tions. They are the product of carelessness or ignorance, or 
both. They have no legitimate excuse under heaven. 

In Fig. 67y 2 , A is the eye of a spectator looking at a screen, 
with four more or less concentrated points of light glaring 
into his eyes, viz. : two "side lights," a clock light and an exit 
sign located beside the screen and unintelligently illuminated. 

If it is desired that a clock be located on the theatre front 
wall its face may be illuminated acceptably, and absolutely 
without glare, as follows : 

From a photographer secure a sheet of dull black paper, 
such as comes wrapped around photographic plates. Cut 
it circular and the size of the clock face. Have a painter 
paint numerals (Roman) from one to twelve, in their proper 
place. Cut a half-inch diameter hole in the center for the 
clock hand post. Split the paper from outside to center hole 
between any two figures, which enables you to slip the black 
face under the clock hands without disturbing them, where- 
upon it may be attached to the clock face, with numerals in 
correct position, by means of a few spots of Le Page's glue, 
without in any way injuring the white clock face, because 
upon removal of the black face the glue may be moistened 
and washed off. 

Now paint the hands dead white, or affix to them false 
hands made of light white cardboard, which may be 


done by means of a white thread. Make the paper hands con 
siderably wider than the original hands. 

Now suspend an incandescent lamp in a can, or suitable box, 
and in one side cut a hole just barely large enough to let 
out a circle of light of sufficient diameter to illuminate the 
clock face, AND NOTHING ELSE. Or, better still, affix in 
a hole in the side a small lens which will project a circle 
of light covering the clock face AND NOTHING ELSE. 

The audience can read the time from white hands on a black 
face very much more readily than from black hands and a 
white face, and your glare spot will have been entirely elim- 

EXIT LIGHTS can be made conspicuous, and at the same 
time absolutely non-glaring, as follows: Paint the letters 
EXIT in red letters of suitable height, and outline them in 
black, so that only the letters show — which is exactly what 
is wanted. No official with a grain of sense or an atom of 
knowledge concerning theatre lighting can or will object to 
this, provided you do it intelligently. The capable official will 
commend the plan. 

You may make your present black-letter-on-light-red- 
ground glaring exit signs both efficient and harmless by shov- 
ing in a sheet of DARK RED glass between the light and the 
letters, or two sheets of light red glass if the dark red cannot 
be had. It will cost you a few cents, yes, but will add im- 
measurably to the comfort of the eyes of your audiences. 

CAUTION. — Don't imagine that because no one complains, 
your glare spots are unobjectionable. The theatre patron 
does not blame the right thing when his or her eyes hurt. 
He does njot know or realize the seat of the trouble, and. 
blames it on "the pictures/' But the point is that he or she 
stays away from picture theatres because "the pictures hurt 
my eyes," or else go to another theatre where "the pictures 
don't hurt my eyes" because the man there is on to his job 
and does not tolerate glare spots. 

WARNING.— Do NOT use an exit sign with black letters 
on a glass painted red. The paint may be O. K. at first but 
soon begins to scale off, and a glare spot is thus gradually 
set up before you realize it. USE RED GLASS ONLY. It 
may be had of or obtained for you by any dealer in photo- 
graphic supplies. 

FLAT SURFACES— LOCATION.— Whatever the surface 
of the screen be composed of, it is olain that it should be as 



nearly as possible perfectly flat, without wrinkles, bumps or 
uneven places, and that its color and "brilliancy" should be 
precisely the same for every portion of its surface. The 
screen should always set as nearly as possible with its center 
level with, and in line sidewise with the projector lens. This 
latter condition is practically never possible of accomplish- 
ment where two projectors are used, since one or the other 
or both must necessarily set slightly to one side of the cen- 
ter of the screen. 

OUTLINING THE PICTURE. — No matter what type of 
screen is used, the picture should invariably be outlined in 
some very dark non-gloss color. This outline should be at 
least 2 feet wide, top, bottom and sides, and 3 or 4 feet is 
better. The outline should extend slightly into the picture. 
In other words the picture should overlap slightly on the 
outline, say one or 2 inches all around. 

The reason for this latter recommendation is that it serves 
to greatly minimize the effect of any movement of the pic- 

Figure 68. 


ture as a whole on the screen ; also it serves to conceal any 
vibration there may be in the projector aperture itself. 

The effect of the outline is to secure added contrast for 
the picture. A picture projected to the center of a white 
v screen without an outline has not anything like the sharp, 
pleasing contrast which the same picture has been projected 
to a white screen properly outlined by a dark border, or a 
border of dead (non-gloss) black. 

Of late the tendency has been to use a neutral tint, usually 
of gray, instead of black, with which practice we have never 
entirely agreed However, some very competent men hold it 
the better practice and possibly they are right. It is neither 
hard or costly to test the matter by hanging some cloth, black 
and of various shades of gray. One claim is that a sharp 
marginal line tends to emphasize the fact that it is a picture. 

One screen authority says : "The matter of too strong a 
contrast between picture on screen and surrounding masking, 
etc., is the same as the matter of too strong a contrast be- 
tween highlights and shadows in the screen picture itself, and 
is the basis of the theory on which 'halftone* screens for use 
with 'high intensity' arcs are constructed, i.e., tinting the 
screen to absorb blues and violets, so that the highlights will 
be subdued in order that the contrast between highlights and 
shadows will not be too pronounced." 

In Fig. 68 we see a characteristic moving picture theatre 
screen setting, with the screen outlined in black. In former 
books we have invariably recommended non-gloss black for 
the picture outline, but while this is ideal in some ways, it 
frequently happens that black will not harmonize with the 
other screen surroundings. In view of this fact, this par- 
ticular recommendation is modified to the extent that any 
reasonably dark non-gloss color will serve fairly well as a 
picture outline. 

The Eastman Kodak Company laboratories have made cer- 


tain rather exhaustive experiments, seeking to determine the 
best methods to pursue in the matter of screen surroundings 
and picture outline border. Concerning the latter they say : 

"Some observations were made during the course of the 
experiments which will probably upset some existing con- 
ventions and increase the comfort of the motion picture 
patrons. For instance, the black velvet frame which fre- 
quently surrounds the screen is found undesirable and a 
neutral gray is suggested in its place. The reason is simple. 
Suppose the illumination of the strongest highlight of the pic- 
ture is 10 foot-candles. Under these conditions the brightness 
of the black velvet frame would be found to be about 0.001 
foot-candles. This makes the ratio of the two or the bright- 
ness contrast equal to 1 to 10,000. This contrast is beyond the 
power of the eye to record and results again in overtaxing the 
process of adaptation. By using a material with a higher 
reflecting power than black velvet, the contrast between the 
screen and the frame may be brought within the range of the 
eye. If the brightness of the frame is raised to 0.02 foot- 
candles, the contrast between the strongest highlight in the 
picture and the frame is 1 to 500. Scientists say that this is 
about the limit of contrast which the eye can endure with 
comfort. In general, therefore, the black velvet frame should 
not be used but, in its place, a material which has a reflecting 
power sufficient to raise the apparent brightness of the frame 
to something like 0.02 foot-candles." 

While we must pay due respect to the opinions and findings 
of such high authorities, still we maintain that some effects 
very difficult to improve upon have been had with black bor- 
ders. We believe, however, that probably as good effects, inso- 
far as concerns the picture itself, may be had with colors other 
than black, and that those colors will usually harmonize with 
the surroundings considerably better. 


Illustrating 1 how a wide, stiff splice pushes the Simplex intermittant sprocket 

shoe away and does not permit of the film seating properly on the sprocket. 

(See page 270) 

Another striking illustration of the failure of wide, stiff or buckled splices 
to seat properly on sprockets. This one will probably cause the film to 
climb the sprocket with resultant more or less serious damage and a pos- 
sible stoppage of the show 
Courtesy Earl Dennison and Paramount. 


"The selection of the material will depend upon a number 
of factors: the illumination of the theatre and the distance 
of the screen behind the front of the stage being the prin- 
cipal ones. For experimental purposes in the laboratory it 
wassfound that covering the black frame with white mill net 
was quite satisfactory. Such an expedient will not in general 
be found satisfactory in practice since this material will 
undoubtedly fail to harmonize with the elegance and richness 
of finish frequently found in the modern motion picture 
theatre. In many cases the screen area is surrounded by 
drapings of silk, velvet, or other fabrics and in such cases it 
is suggested that a fabric harmonizing with the general 
decorative scheme be used as a draping immediately around 
the screen area, the color that will give a satisfactory result 
being such as in. ordinary terminology is referred to as a 
rather dark gray. A very pleasing result was obtained in an 
experimental installation by the use of a screen frame covered 
with a warm-gray burlap such as is used for wall coverings. 
In case the decorative scheme is carried out not by the use 
of fabrics but by the use of painted surfaces, the frame 
should be made by use of a rather dark gray paint. Of 
course, it should be understood that in case a true gray does 
not harmonize well with the decorative scheme of the interior 
some rather dark color tone (including colors usually referred 
to as warm or cool grays) may be used with advantage. In 
any event, small samples of fabric or small panels painted 
with various colors should be tried by placing them tem- 
porarily in position near the screen and the final choice made 
when a material or paint is found having a reflecting power 
such as to make the frame appear of the correct brightness." 

We are unable to altogether agree with this. We see no 
reason why the screen border should be visible to the eye 
at all. In fact, the less visible it is, it seems to us the better 
is the condition. However, we yield all due respect to the 
views of the gentlemen who made these experiments and 
pass the matter along to you for consideration, with the note 
that we still adhere to our recommendation of either a dead 
black, or at least a very dark non-gloss screen border. 

We .commend the booklet issued by the Eastman Kodak 
Company, entitled "The Motion Picture Theatre, Its Illum- 
ination and the Selection of the Screen." 

It is a twenty-four-page, paper-covered phamplet. It will 
be sent gratis to projectionists who say in what theatre they 
are employed. Get it for your library. 


mediate screen surroundings, it is highly important that they 
be absolutely non-gloss in character, and not too light in 
color. This is of very great importance indeed where an 
orchestra of considerable size is located immediately in 
front of, or near the screen. Very great injury is often done 
to the screen result by the light reflected by the white sheet 
music to the light colored semi-gloss screen surroundings, 
and by them re-reflected back to the screen. Where there 
are from 20 to 40 orchestra lights it requires no great wisdom 
to understand that a vast amount of reflected light may reach 
the screen in this way, all of which not only adds extraneous 
light to the lighted portions of the picture, but also to the 
shaded and black portions, causing the latter to appear a 
dirty gray color, instead of pure black. The direct effect of 
this is to very greatly injure the contrast of the picture. 

PAINT STAGE FLOOR.— Where a screen is set well back 
on the stage of a theatre having a balcony, it is highly im- 
portant that the floor of the stage be either covered with 
non-gloss black cloth or painted non-gloss black. Unless 
this is done the light reflected from the floor will be very 
annoying to the eyes of the patrons seated in the balcony or 
at least it will detract from the contrast and beauty of the 

SIZE OF PICTURE.— (Also see "Definition and Magnifica- 
tion," Page 245.) — The size of the picture has been the sub- 
ject of perhaps more argument than any other one thing in 
connection with the motion picture theatre auditorium. This is 
because of the fact that a number of things are directly in- 
volved in the matter. The author of this work has always 
opposed large pictures. He has repeatedly said, and does 
still say, he has never yet seen a theatre in which he con- 
sidered there was any real necessity for a picture of greater 
width than 18 feet, and those theatres which really require a 
picture wider than 16 feet are very rare indeed. 

The author has stood in Madison Square Garden, New 
York City, at a distance of more than 200 feet from a 16-foot 
picture and has been able to read both the titles and sub- 
titles with very little effort. He was able to follow all the 
action of the photoplay without the slightest difficulty. 

Mazda projection has served to demonstrate the fact that 
what was formerly believed to'be an impossibly small picture 
is really plenty large enough for the ordinary theatre. We 


have watched the projection of a 12-foot picture in a theatre 
seating 1,500, with the audience apparently thoroughly 

If the screen end of an auditorium or theatre is properly 
designed or decorated so that it does not present a great un- 
broken expanse but is broken up into small a eas that will 
harmonize with the smaller picture the patrons will pay no 
attention to the size of the screen. Small screens are con- 
spicuous because they are generally out of harmony with the 

We will now endeavor to discuss this matter of picture size 
in some of its more important details. In the first place the 
size of the picture has directly to do with the value of the 
front rows of seats, because if the picture be too large in 
proportion to its distance from the front row of seats, then 
those occupying the front seats will be subject to heavy eye 
strain, besides having a highly unsatisfactory view of the 
picture. The value of these seats will therefore be greatly 
reduced. The eye strain will be caused by two separate 
things, viz. : first, the picture will not appear in sharp focus, 
which in itself causes heavy eye strain; second, if the person 
be seated too close to a picture of large size the movement 
of the eye in following the action on the screen through the 
wide angle involved will set up terrific strain. 

After a great deal of study, observation and consideration 
we have concluded that the front row of seats should never 
be closer than 20 feet from a 16-foot picture, and in order 
that the same angle of view be maintained it is necessary 
that one foot 3 inches of additional distance be added for 
each additional foot of picture width. 

In exact opposition to the foregoing it must be remem- 
bered that if the picture be too small and the auditorium be 
a long one, then eye strain may be set up for those who 
occupy the rear seats, though this ordinarily only holds good 
where the rear seats are an unusually great distance from 
the screen. 

We think we may safely say that there will be no appreci- 
able eye strain for those of normal eyesight if no seats be a 
greater distance than 100 feet from a 16-foot picture. As a 
matter of fact a great many people would experience no eye 
strain at a considerably greater distance, but we believe 100 
feet may be accepted as a fairly safe guide for the average 
eye. Those who experience eye strain at that distance can of 
course secure seats nearer the screen and we must remember 


that the smaller the picture the more brilliant it can be made, 
hence the greater distance it may be viewed without eyestrain. 

The smaller the picture, within reasonable limits of course, 
the more valuable the front rows of seats become. 

Except under very unusual circumstances we strongly 
recommend that no picture exceed a maximum width of 18 
feet. As to the minimum, it may be fairly said that a 10-foot- 
wide picture is about as small as any one would care to use 
for theatrical purposes. 

Note — The following is the comment of a man whose knowl- 
edge of screens and matters pertaining thereto I have great 
respect for. It is set forth because it sets forth an undoubtedly 
very important angle of the "picture size" mater. 

"I cannot see how screen size would have much to do with 
eyestrain, though the size of objects pictured on the screen 
might have some bearing on strain. For instance, in a 'close 
up' view_ a man's face is sometimes so large that it fills the 
wmole screen. There would certainly be no eyestrain in 
viewing a 12-foot screen under this condition, even at a 
distance of 200 feet. However, an object that is actually 10 
feet high might be photographed in such a way that it meas- 
ured only 2 feet high, even on a 16-foot screen. In this case 
there might be eyestrain even at 50 feet. 

"It is all a matter of proportion, so that if the picture of a 
man who is 6 feet tall is projected on a screen so that his pic- 
ture measures 6 feet on the screen, there will be no more eye- 
strain in viewing this man's picture at 100 feet (or any other 
distance) than there would be in viewing the catual man at the 
same distance, light and other matters being equal." 

ture size is only had by increasing the magnification of the 
film photograph in its image at the screen, with the natural 
result that definition is impaired, it very seldom being per- 
fectly sharp in the photograph itself to begin with. See mag- 
nification of defects, page 246. 

membered that as the size of the picture is increased the 
amount of light necessary to maintain its brilliancy per unit 
of area increases rapidly. 

The magnification of the film photograph is in any event 
enormous. Its linear magnification may be found by multi- 
plying the width of the image in inches by 32 and dividing 
that result by 29. The result will be the number of times the 
film photograph is magnified in the width of the screen image, 
the projector aperture being 29/32 of an inch wide. The fol- 
lowing figures are interesting. 

Table No. 9. 

Size of Picture. Surface Area. Magnification. 

9x12 108 Square feet 158.88 diameters 

12x16 192 Square feet 211.84 diameters 

15x20 300 Square feet 264.80 diameters 


If you were to cover a 16-foot picture with film photo- 
graphs just the size of the projector aperture it would re- 
quire 44,944 of them to do it. And since there are 16 photo- 
graphs to the foot of film it would be necessary that 2,788 
feet of film be cut up to supply the photographs. 

Table number ten gives the height and area in square 
feet of pictures from 10 to 20 feet wide, by one foot steps. It 
also gives the percentage of illumination or brilliancy per 
unit of area each size would have as compared with the 10- 
foot-wide picture, amount of light passing the revolving 
shutter of the projector being the same in all cases. 

Table No. 10 is enlightening. We find that by increasing 
picture size from 10 feet to 14 feet we have decreased its bril- 
liancy by 49 per cent., and that a 16-foot picture will require 
61 per cent, additional light to be as brilliant as the 10-footer. 

Of course the table assumes that the same percentage of 
the total light reaches the screen in every case. For prac- 
tical purposes the percentage column really shows the per- 
centage of area in reverse. For instance : the 10-foot picture 
has just 25 per cent, of the area of the 20-footer; the 10-foot 
picture has just 39 per cent, of the area of the 16-footer, and 
so on. 

Table No. 10. 

eiR"ht in 

Width in 



Area Sq. Ft. 



x 10 


100 pi 

er cent 


x 11 





x 12 





x 13 





x 14 





x 15 





x 16 





x 17 





x 18 





x 19 





x 20 




MAGNIFICATION OF DEFECTS.— One phase of picture 
magnification, or picture size, should not be overlooked, 
especially by the exhibitor showing old films, viz. : the effect 
of magnification on defects. As has been shown, the magnifi- 
cation of the original photograph in any picture of theatrical 
size is terrific. But in the larger sizes it is, of course, very 
much greater than in the smaller. 





.2 w 





w 3 






Applied to defects, suppose you have film which has a good 
deal of "rain." If the actual scratch in the film itself be 
.015625 (1/64) of an inch wide it will appear as a mark ap- 
proximately 2.5 inches wide in a 12-foot picture, but in a 20- 
footer it will be widened to a little more than 4 inches. We 
thus see that as the picture width is increased all defects in 
the film photograph are magnified and made more visible, but 
especially this applies to rain, it being more or less continu- 
ous throughout old film. 

It may also be noted, in passing, that any side motion of 
the film ir the aperture will be magnified on the screen in 
the same proportion. If the film moves sidewise .015625 (1/64) 
of an inch in the aperture, the 12-foot picture on the screen 
moves about 2.5 inches and the 20-footer moves a little more 
than 4 inches. 

SCREEN TINTING. — Screen tinting is a thing which we 
have not agreed with in the past, nor do we now agree with 
it as it is generally practiced. Experiments, however, seem 
to have shown that a screen may be so tinted that the harsh 
lighting will be toned down, and the results largely improved, 
without any compensating damage. 

As generally done, even the most slight introduction of tint 
kills the white entirely. If tinting is practiced with the best 
possible advantage, it must be by some method by means of 
which the whites are not entirely killed, while at the same time 
the harshness of the light is subdued. This is claimed to be 
entirely practical. One screen .manufacturer explains the 
methods by which the result is attained as follows : 

For the viewing or reflecting surface of the screen we use 
a very finely woven cotton fabric, which has been previously 
treated with certain chemicals in order to impart to it certain 
characteristics. On the back of this cotton sheet we spread a 
coating of specially prepared and tinted rubber, in a semi- 
liquid state. Rollers under six ton pressure forces this rubber 
into the mesh of the cotton, so that it shows, under microscopic 
examination, on the face side of the fabric in the form of 
tiny tinted points of rubber. The actual screen surface then 
is a very fine mosaic made up of white threads and colored 
points of rubber. 

It is a well known fact that white reflects all colors. In 
other words it is non-selective. On the other hand there are 
many substances which are selective in the sense that they 
absorb certain colors and reflect others. With this knowledge 
as a basis we make a spectrum analysis of the light (high 
intensity, regular arc, mirror reflector type arc and incande- 
scent) for use with which the screen is being constructed, to 
determine in what respect the light differs from sunlight — the 
perfect light. 

This is our guide as to just how the screen should be tinted, 
or rather as to just what color the rubber filling should have 


in order that the rays which cause glare and harshness may 
be absorbed. 

It will be observed that by this process the whole surface 
is not tinted. No matter how dense the tinting may be, fully 
fifty per cent of the surface is still pure white. 

The foregoing is given space as an interesting description 
of a screen manufacturing process. We neither vouch for, 
or question its value. 

Personally we believe the right place for softening the light 
is at its source. We believe the time will come when by 
proper chemicalization of the carbons the tone of the light 
will be softened and controlled, so that screen tinting will no 
longer be either necessary or advisable. One of the greatest 
disadvantages of screen tinting is found in the fact that a very 
large percentage of present day films are tinted, and the intro- 
duction of color in the screen surface cannot possibly be 
made to "fit" into a dozen or more tinting colors to advantage. 

Heretofore we have not favored tinting because of the fact 
that the introduction of color into the screen surface auto- 
matically operated to reduce screen brilliancy, but light sources 
have become so very powerful, and lens systems so greatly 
improved, that a slight reduction in brilliancy caused by tint- 
ing may easily be compensated for, because almost unlimited 
light is available to the modern projectionist. 

Experience has amply proven that the location of the 
screen at the end of the auditorium where the audience 
enters, with the projection room at the opposite, or rear end 
of the auditorium, is very bad practice indeed. Its effect is 
not good in any way, and when we consider the fact that the 
modern projection room is absolutely fireproof, and that if 
the port shutters be properly constructed and fused, and the 
projection room be properly ventilated, no evidence of any 
fire which may occur therein will be visible in the auditorium, 
we readily see that absolutely no element of safety is served 
by a front of the house screen location. Local authorities 
will do well to pay more attention to the proper construction 
of the projection room, the proper construction of its port 
fire shutters, the proper location of the fuses controlling the 
port fire shutters, and the proper ventilation of the projec- 
tion room, instead of evolving such utterly useless, not to 
say foolish schemes as placing the screen at the entrance 
end of the auditorium. 

is located on a stage, and the theatre is used for motion 
pictures only, the screen should in any event be located far 
enough back so that there will be a minimum distance of 20 


feet between the front row of seats and the screen, if the 
picture be 16 feet or less in width. See Page 244. For each 
added foot of picture width there should be an additional one 
foot and three inches between the front row of seats and the 
screen in order to maintain the same viewing angle from the 
front seats. 

If the depth of the house from the proscenium to the rear 
row of seats be not to exceed 75 feet, it is always very much 
better to set the screen back on the stage as far as it can be 
placed without interfering with the view of the screen from 
the extreme front side seats. Those in the rear seats will 
still be close enough to have a good view of the screen, 
while those in the front rows and at the side of the audi- 
torium will have a vastly improved view over what it would 
be if the screen were further front. 


theatres use a mixed performance of vaudeville and feature 
pictures. Where this is done it is very much better that the 
pictures follow the vaudeville, because the screen may then 
be placed at the rear of the stage, where those occupying the 
front rows of seats will have at least a fairly good view with- 
out serious eye strain. If the pictures precede the vaudeville, 
then it will usually be necessary to place the screen near the 
front curtain, in what is known in theatrical parlance as 
"one," in order that the stage may be set for the first vaude- 
ville act while the picture is running. This sets up a very 
bad condition for patrons occupying the front rows of seats. 
They will experience heavy eye strain by reason of the fact 
that the picture will not appear to them in sharp focus, and 
for other reasons set forth on page 244. 

In theatres using a combined vaudeville-picture bill there 
is, except in a comparatively few isolated cases, an astound- 
ing indifference shown to the proper presentation of the 
picture, although the picture in many cases supplies fully 
half the bill. The screen more often than not is badly 
located, and very often is to all intents and purposes merely 
a flat sheet of very poorly coated muslin. 

In such houses the screen should be stretched on a sub- 
stantial frame, and should slide up and down in grooves. 
The screen should be properly counter-weighted, as can be 
very easily done in modern theatres. Where this plan is 
used the screen will always be precisely in the same place, 
and currents of air will not move its surface. 

We have often sat in a high-class vaudeville-picture thea- 


tre charging top hole prices, and have watched a scene per- 
haps containing huge buildings sway backward and forward, 
because the management had failed to properly support the 
screen by a framework, but were using a painted drop 
weighted at the bottom by a wooden strip, with result that 
every time there was an extra strong current of air the 
screen moved to and fro. 

TRAVELING EXHIBITORS may carry a painted cloth 
screen or metallized surface screen successfully, always pro- 
vided it be rolled face inward on a round wooden rod not less 
than 3 or 4 inches in diameter, and further provided that a 
clean muslin sheet be spread over the surface of the screen 
before it is rolled, this in order to protect the surface from 
dirt which may accummulate on the back of the screen. A 
painted screen rolled on such a support (which latter may be 
made up by nailing thin wooden strips, such as lattice work 
is made of, around round wooden end and center supports) 
will not be very bulky and its surface will not crack. Of 
course if the screen is to be shipped by express, then an 
outer wooden covering would necessarily have to be made 
for it. 

CAUTION. — The screen should be rolled as tightly as pos- 
sible in order to prevent the surfaces rubbing against each 
other in transit. 

FIREPROOFING SOLUTION.— Any screen or other fabric 
may be fireproofed by thoroughly saturating it with am- 
monia phosphate, mixed in the proportion of one pound to 
one gallon of water. In applying the solution to a cotton 
screen it should first be tightly stretched on a frame and the 
solution applied with a cheap paint brush, prior to the appli- 
cation of the glue sizing. Let the fireproofing dry thoroughly 
before applying the glue size. 

Fabric which has been thoroughly saturated with ammonia 
phosphate solution will char, but it will not and cannot be 
made to blaze. If you hold a lighted match against the 
fabric the result will be a hole charred in the cloth — that is 
all. There is nothing in ammonia phosphate that will in any 
way injure the fabric. Wood thoroughly soaked in the solu- 
tion is made fireproof 'in the sense that it cannot be made to 

STRETCHING THE SCREEN.— The wide use of metallic 
surface screens, many of which are constructed of heavy 
cloth or canvas, makes it very difficult to stretch them tight- 



Figure 69 

ly, though tight-stretching is necessary since with a semi- 
reflective surface every wrinkle or uneven place will show 
There is nothing better for this purpose than what is 

known as the "artist 
frame." It is very much 
superior to any home- 
made arrangement, and 
may be purchased from 
almost any screen 
manufacturer for less 
than it would cost an 
exhibitor to make it. It 
is simple, and we be- 
lieve quite satisfactory. 
It may be shipped 
knocked down, and the 
process of putting it 
together is one which 
can be readily per- 
formed by any man of ordinary intelligence. 

Begin to put the frame together by laying it bottomside 
up, on a floor, or other flat surface. After the corners are 
bolted together see that the whole frame is exactly square. 
This may be tested by measuring diagonal corners. If the 
distance from diagon- 
ally opposite corners 
is equal the screen, as 
a whole, is square. 
Next, put on the back 
braces and then turn 
the frame over, or set 
upright in place. The 
various steps in the 
process are shown, in 
their order, in Fig. 71. 
CLOTH. — The cloth 
should be rolled up so 
that the edge that goes 

to the top unrolls first. It may be put on either with the 
frame standing up or lying down. Standing the frame up- 
right is the best plan, however, because the cloth will partly 
stretch by its own weight, and the whole job will be more 

Figure 70 



easily and better done. A good start is a long step toward 
success. Lay the roll of cloth on a level floor, unroll a foot 
or two, and stretch a chalk line to determine whether or not 
its edge is perfectly straight. Trim it if necessary to fit the 
chalk line. Now make a chalk line across near the extreme 
edge of the top of the frame, on the front side, where the 
cloth is to be tacked. The straight top edge of the cloth and 
the line on frame are placed together and the cloth is tacked 
fast, thus insuring a good, straight start. 

TACKING ON CLOTH.— Place the tacks about two inches 
apart. A thin tack with a large, flat head is the best. If the 
frame is placed upright a piece 
of cheese cloth should be looped 
and nailed to the frame on each 
end, to hold the roll of cloth in 
position while the top edge is 
tacked in place. Start at the 
center of the top, and tack both 
ways along the chalk line, until 
within about three or four feet 
of the corner. A single tack 
will hold each corner in position 
until you are ready to tack 
corners. Now unroll cloth slow- 
ly and carefully, keeping it 
stretched at all times. Stretch 
and tack the bottom of screen, 
beginning at center and working 
again to within three or four 
feet of each corner. Next tack 
one side from center to within 
a short distance of corner, and 
then tack and stretch the cloth 
on the other side, after which 
finish up the corners. 

In tacking any cloth screen 
always begin at centers of top, bottom and each side, and 
finish corners last. 

If the work is done carefully the surfaces will be almost 
entirely free from wrinkles, and where a light cloth is used 
and well stretched by hand a very even surface is possible 
on a common hand-made frame. The artist ffame we are 
describing is provided with finishing strips which are added 
in order to cover up the tacks and raw edge of the cloth, 

Figure 71 



which helps the appearance very much. Beveled stretcher 
strips are then pushed down between the cloth and frame 
from the back, giving the appearance of a bevel around 
the edge on the face side. This gives a handsome, finished 
appearance to the screen generally. 

In most cases the cloth is free from wrinkles when the 
stretcher strips are put in position, but to provide for fur- 
ther stretching lag bolts are placed in the frame which, 
when screwed in, push out the stretcher strips still farther, 
so that the screen can be made as tight as a drumhead. 
The artist frame is always good property, as it can be used 
again for new cloth. Those exhibitors who use metallized 
screens should renew them at least every two years. Many 
metallic screen surfaces lose their brilliancy in even less 
time, and often those of inferior quality will become dull 
within a few months. Fig. 69 shows front of finished screen. 

SIDE VIEW DISTORTION.— The effect of viewing the 
screen at heavy angle should be, but apparently is not, 
thoroughly understood by architects. To a person seated at 
an extreme side angle to the screen all figures thereon ap- 
pear to be abnormally tall and very thin. The explanation 
for this is very simple. 



Figure 72. 


In Fig. 72 we view a theatre screen E F, A B an object 
in the picture on the screen, and C and D the eyes of two 
spectators, one seated directly in front of and the other at 
a wide angle to the surface of the screen. C of course sees 
object A B at its full width, but, remembering that the ob- 
ject on the screen is only an image, hence absolutely level 
or flat, it is evident that D will get the effect only of width 
B G. This explains the apparent lack of breadth in the 
object. The effect of abnormal tallness is, however, 
partly due to the foreshortening of the width of the 
object. We have been accustomed to seeing a man or 
woman of a given height have, within certain limitations, a 
certain given breadth. If we foreshorten the breadth un- 
naturally the effect is to give the impression of greatly 
added height. 

Another reason for the apparent tallness is that it is real- 
ity. The figures on a screen are often very much taller than 
in real life. This is not realized because they have the cus- 
tomary proportions. If, however, the breadth be foreshor- 
tened until a figure ten feet tall has only the width of a 
normal man or woman the effect is a ridiculously tall, thin 
caricature of the original. 

The lesson taught by this is that patrons should not be 
seated at too wide an angle to the screen. Architects may 
readily determine exactly what the effect in foreshortening 
of width will be at any given angle by applying the simple 
process shown in Fig. 72. 

of the picture and its outline caused by the projection lens 
being out of center with the screen is commonly termed 
"keystone effect," because of the fact that when the pro- 
jection lens is considerably above the center of the screen 
(the condition most commonly met with) the picture outline 
assumes, in greater or less degree, according to the condi- 
tion, the shape of an inverted keystone. 

This is illustrated in Fig. 73, in which A is the projection 
lens, B C the screen, E F a horizontal line perpendicular (at 
right angles) to the surface of the screen at its center, B D 
the position screen B C must assume in order that its sur- 
face be perpendicular to (at right angles with) the axis of 
projection, D C the distance lower margin of the light beam 
must travel in order to reach the screen surface in excess 
of distance travelled by the upper margin. The solid lines 
of H show the resultant shape of picture on the screen, and 


the broken lines what the picture should be and would be 
were lens A located at E, or screen B C in position B D. 
Observe that the picture will be wider at the bottom and 
narrower at the top than it would be were the axis of pro- 
jection along line E F — the projection lens central with the 
center of the screen. Analyzing Fig. 73 we find approxi- 
mately as follows : Distance of projection being 80 feet and 
the width of the picture 16 feet at its center, it would, under 
normal conditions be twelve feet high. Since the picture is 
192 inches (16 feet) wide, the beam must spread out 192 
inches divided by 80 feet equals 2.4 inches for every foot of 
projection distance. Distance D C is 3 feet 8 inches, or 3.66 
feet, hence with the screen in perpendicular position the 

Figure 73, 

beam must travel 3.66 feet further in order to reach the 
lower edge of the screen than to reach its top. It there- 
fore follows that the beam will be 3.66x2.4 inches=8.786 
inches wider at its bottom than at its top, and proportion- 
ally all the way up and down the picture, thus producing the 
shape known as "keystone," shown by solid lines at H, Fig. 

Fig. 74 gives us the number of inches drop in projection 
per foot of distance when the angle is 12 degrees. It is 2.55 
inches. If it is proposed to have the lens a certain height 
above screen center, and a given distance from the screen 
(distance of projection) it is only necessary to multiply the 
projection distance, in feet, by 2.55 to know whether the 
angle will be more or less than 12 degrees. If the result is 
greater than the proposed height of the lens above screen 
center, measured in inches, the angle is less than 12 degrees. 



For instance: Proposed projection distance 60 feet. Pro- 
posed height of lens above screen center ten feet (120 
inches). 2.55x60==153, hence at 60 feet the lens must be 153 
inches above screen center to produce a 12 degree pitch. 
Its proposed location is less than that, hence the angle will 
be less. 

of Motion Picture Engineers has set 12 degrees from a hori- 
zontal line passing through the center of the screen (it is 
not so stated, but presumably that is what is meant) as the 
maximum permissible angle of projection. This, as is shown 
in Fig. 74, means a horizontal rise of 2.55 inches each foot 
of projection distance. 

Let us examine the matter. Assuming a 96-foot projec- 
tion distance, a 12-degree angle and a 16-foot picture, we 

Figure 74. 

find that the beam will spread just 2 inches per foot. Using 
a similar angle and the same size picture, but a 50-foot 
projection distance, we find the spread of the beam to be 3.8 
inches per foot, but since both the angle and picture size 
remain constant, distance D C, Fig. 73 f will remain un- 
altered, and since the spread per foot of the beam is greatly 
altered by the changed distance of projection, the resultant 
distortion of the picture will be very much greater on the 
short projection distance than on the long. Hence we say 
that angle of projection is not always a safe guide. 

In our opinion the only reliable guide to permissible pro- 
jection angle is the amount of distortion a given condition 
will produce, and up to this time no competent authority has 
undertaken to say what amount of distortion may be toler- 
ated. It must be remembered that the resultant distortion 



is present all over the image. It is not confined to the pic- 
ture outline, but alters the relative width of things in dif- 
ferent portions of the picture, as well as making the whole 
picture and everything in it abnormally tall. In fact this 
latter is the worst feature of the distortion. Outline distor- 
tion may be corrected by filing the aperture plate opening. 
Distortion of width of objects in the picture usually is not 
very noticeable, but added height is very readily discernible. 
Fig. 75 illustrates the effect of projection distance on 
projection angle, height of projection lens above screen re- 
maining the same. If the lens be located 25 feet above 
screen center, and the projection distance 40 feet, the pro- 
jection angle will be 32 degrees. With the projection dis- 
tance 80 and 120 feet respectively, the projection angle is 


Figure 75 

reduced to 17 degrees and 20 minutes, and 11 degrees and 46 

It is the opinion of the author that picture height offers 
the best and safest guide to permissible projection angle, 
and that any projection room location which increases the 
normal height of the picture by more than 5 per cent, is ob- 
jectionable and should not be tolerated. 

It will be observed that this permits an increase of the 
height of a sixteen-foot-wide picture by a trifle more than 
seven inches, so that at 80 feet projection distance a 15-de- 
gree angle would be within the limit by .2 of an inch. This 
would automatically add a maximum of 5 per cent, to the 
height of all objects on the screen, including the actors, so 
that a six-foot man who happened to appear just life size in 


a normal size picture would appear as being 6 feet, 3.6 
inches tall; also his head would be smaller in proportion 
than his feet, though this latter, if confined within this 
range, will not injure the results perceptibly. 

While the following figures are not exactly correct, they 
are nearly enough so for our purpose. 

Given a projection distance of 80 feet, the following pro- 
jection angles will increase the height of a 16-foot-wide pic- 
ture as follows : 

Angle of 10 degrees increases height 4.5 inches. 
Angle of 15 degrees increases height 7 inches. 
Angle of 20 degrees increases height 13 inches. 
Angle of 25 degrees increases height 20 inches. 
Angle of 30 degrees increases height 32 inches. 

average exhibitor, and very many unthinking projectionists, 
believe that so long as the sloping sides of a distorted pic- 
ture are made perpendicular, which may be done (see Filing 
the Aperture below), there is no remaining evil except 
the added difficulty of obtaining sharp focus all over the 
screen. They base their belief on the fact that the theatre 
patrons do not know or realize that the picture is distorted, 
hence no harm is done. 

This is fallacious reasoning. Admitting the fact that if the 
sides of the distorted picture be made perpendicular, the 
audience, having nothing as a basis for comparison, prob- 
ably will not know the distortion is present, the fact remains 
that the distorted picture is not nearly so pleasing to the 
eye as the undistortecfr one, or the one only slightly distorted, 
and since theatre patrons pay admissions to theatres in or- 
der that they may be amused and entertained, it follows that 
the more pleasing the appearance of the picture as a whole, 
aside from its merits as a play, the better satisfied the 
patron will be, and the better satisfied the patron is, the 
more he is likely to patronize the box office frequently. 

FILING THE APERTURE.— Side lines of Keystone may be 
made parallel by rilling side of aperture with hard solder or 
procuring special aperture plate from projector manufacturer 
and filing sides to suit. First project light to screen and make 
mark on screen at lower ends of top corner curves. Then 
remove aperture and fill in sides or substitute special aperture. 
Next place a metal plate over the lamphouse cone, in the 


center of which is a hole about ^ of an inch in diameter. 
Next hang cords at the screen so that they will pass down 
exactly over the marks you have made at the bottom of the 
upper corner curves. Now strike an arc, and, with the light 
through the hole in the plate over the cone to guide you and 
show you the exact effect of every move, file out the aper- 
ture sides until the light comes exactly to the lines on the 

The light enables the worker to watch the exact effect of 
every stroke of the file. He is thus enabled to do a very ac- 
curate job. It is necessary to be extremely careful in filing 
because if you get a bit too much metal off at any stroke of 
the file it means the job must all be done over again. 

Keystone effect is invariably accompanied by a greater or 
less tendency to out of focus. This is especially true if it be 
side keystone, since the picture is wider than it is high. It 
is caused by the fact that a projection lens is presumed to 
focus at a given distance, (Fig. 36 D), and since with distortion 
of the kind we have described the distance to the screen 
varies, it will readily be seen that a strain is placed on the 
powers of the projection lens in the matter of focusing. The 
lens may be given increased depth of focus, or in other 
words may be caused to focus over a greater distance by re- 
ducing its diameter, and this is why large diameter objec- 
tives are a very hard proposition to handle where there is 
any decided tendency to distortion in the way of keystone 
effect. If the projectionist is working under conditions of 
heavy distortion or keystone and is unable to get a sharp 
focus all over his picture, let him try stopping down the 
diameter of his projection lens by inserting a ring of black 
cardboard in the front end of the lens barrel, right up 
against the front factor of the projection lens. In the cen- 
ter of this cardboard cut out a circle say one inch in 
diameter. If this sharpens the picture, then he can know 
where the trouble lies and can increase the size of the hole 
in the stop until the trouble again appears, after which a 
new metal disc with an opening just a little bit smaller will 
serve the purpose. We know of no other means of remedy- 
ing such a condition, so long as the distortion remains, and 
the remedy we have suggested may be quite expensive in 
light, therefore it is a waste of electric current. 


The Spotlight 

THE projectionist is sometimes called upon to operate 
a spotlight, and if without experience may feel 
nervous about making the attempt. This is unneces- 
sary, as the apparatus is simple and easy to manipulate. It 
consists essentially of a lamphouse, similar to the ordinary 
motion picture projector lamphouse, in which is an ordinary 
arc lamp similar to the arc lamp used for projecting motion 
pictures. There is an arrangement by means of which the 
lamp may be quickly advanced or pulled back in order to 
alter the distance of the crater from the lens, since it is this 
act which alters the size of the "spot," or changes it to a 
"flood/' In the front of the lamphouse a single plano-convex 
lens is mounted. There are no other lenses in connection 
with a "spot," as such devices are called. The lamphouse is 
mounted on an upright, which is adjustable in length, so that 
the height of the lamphouse from the floor may be changed 
at will. The upright is supported by a suitable base. On 
some of the smaller spots a small wire coil rheostat is 
mounted, but with most modern spots the rheostat is a sep- 
arate unit. 

The lamphouse is so mounted that it may be swung from 
side to side, or tilted up or down, since by these movements 
the direction of the light beam is directed. 

Roughly this decsribes the old type "spot," which same is 
illustrated in Fig. 76A. In Fig. 76 we have a view of the 
optic end of it. 

The lens usually is six inches in diameter for small spots, 
though larger ones may have lenses eight inches in diameter. 
There is a single plano-convex lens only. 

As said before the size of the "spot" is controled by the 
distance of the crater to the lens. The further away it is the 
smaller the "spot," and the closer it is the larger the "spot." 
If it be shoved close enough to the lens a "flood" will result, 
which may be made to cover the entire stage. 

Spot lights use anywhere from fifteen to seventy-five or 
more amperes P. C, which is taken through an ordinary 
rheostat or motor generator of suitable capacity. 

HANDLING A SPOT.— No especial skill is required to 
handle a "spot." A man of ordinary intelligence should, after 



a few moments practice, be able to cover an actor when 
moving about, and do it well, too. The difficulty, is not in 
handling the device, but in the matter of carbon setting. 
With the old style arc lamp considerable experience and skill 
is necessary to get an approximately round spot which is 
free from ghost. The spot is nothing more or less than an 
out-of-focus photograph of the crater of the positive carbon, 
therefore, unless the crater presents a round surface or cir- 
cumference to the lens, the resultant spot will not itself be 
round ; also faulty carbon setting is likely to produce a ghost 
in the spot. 

The lamp should be given a pretty heavy angle, and the 
carbons should be set much the same as for motion picture 
projection. It is then up to the projectionist to experiment 
with the amount of advancement of the lower carbon tip 



Figure 76A 

with relation to the other, and with the angle of the lamp 
itself, until he gets the condition which produces the most 
nearly round spot, and one free from ghost. 

KIND OF CARBONS.— It will, of course, be necessary to 
use a cored carbon for positive. Any good projection car- 
bon will do. For negative the projectionist may try a cored 
carbon, of smaller diameter than the positive, and a Silver 
Tip and Hold Ark, using the one giving best results. 

Remember that following an actor with a spot is childishly 
simple. The hard job is to get and keep a clear, round spot* 

The spotlight port in the wall should be of minimum size 
local conditions will permit. A color wheel suitable for mount- 
ing on a spot may be had from any supply dealer. It is a 
very necessary adjunct to a spot light. Color slides may also 
be had. 

A. C. SPOT. — We do not advise you to try it, unless you 
use pretty high amperage. It is possible to get a spot with 



A. C. at ordinary amperage, but it will not be a good one. 
Seventy-five amperes is what we would consider as the mini- 
mum amperage for a spot light, if good results are to be 

HIGH INTENSITY SPOT.— The Nicholas Power Company 
is marketing a spot which uses the high intensity lamp. This 
spot is, in many respects, a distinct innovation. 

The base is heavy and has three extensions or "feet," one 
of which is about four inches longer than the others. The 
long leg goes toward the back of the lamphouse. The lamp- 
house is 29 inches front to back, 31 inches from base to top 
of vent cone, and 11^4 inches wide. It is 
made of heavy material, with double-wall 
doors. Adapters are provided, which 
enable the use of anything from a 4 to 
an 8-inch diameter lens. These adapters 
fit into a groove in casing A. In front of 
the lens, also fitting into a groove in cas- 
ing A, is an iris diaphragm with an 8-inch 
opening, constructed entirely of brass. 
In front of the iris are two grooves, 
designed to carry either the support for 
a color wheel, or color slides. The 
observation window in the lamphouse is 
10 inches long by 3 inches wide, and is 
covered with dark red glass. Casing A 
is of grey cast iron. 

The construction is such that once the 
spot is in the desired location it may, if 
desired, be locked there by means of 
wheel B and handle C. By loosening 
wheel B the lamphouse may be swung 
clear around on the pedestal. Handle 
C is a locking device, so that the lamp- 
house may be tilted to any desired angle and locked there. 
With both handles B and C unlocked the spot may be moved 
to any desired location, from on the floor three feet in front 
of or back of the machine, to an angle of probably 75 degrees 

The machine, exclusive of the pedestal, weighs probably 
150 pounds, yet so perfect is the construction and the balance 
that it may be moved in any direction with one finger; also, 
the balance is automatically maintained, regardless of the 
position of the lamp, because when the lamp is pulled back, 
weight D automatically moves forward just enough to coun- 

Figure 76A 



terbalance the weight of the lamp. Handle E controls the 
forward and backward movement of the lamp and the 
weight. Rod F has teeth on its under side and rod G teeth 
on its upper side. Between these two rods is a toothed 
wheel. This wheel is actuated by rod F when the lamp 
moves forward or backward, and this, acting through the 
toothed wheel, moves rod G in the opposite direction, thus 

Figure 76B 

carrying weight D backward and forward to counterbalance 
the weight of the lamp, and thus keeps the lamphouse 
always evenly balanced on the pedestal. 

The spot marks a very great step forward in the produc- 
tion of high-class equipment for theatres. 

versal Stage Lighting Company, better known as Kliegl 
Brothers, 321 West 50th street, New York City, from whom 


spotlight lenses of all sorts may be had, supplies the follow- 
ing valuable data: 

"We are very often requested to supply a lens to give a 
small (5 foot) spot when the distance of projection is long 
It is not possible to handle such requests intelligently unless 
the following information accompany the order: (A) Dis- 
tance lens to stage; (B) diameter lens the spotlight will ac- 
commodate; (C) length of spotlight hood, or greatest distance 
crater can be gotten from face of lens. 

"Our regular 25-ampere stage spotlight, which uses either 
a 5 or 6 inch diameter lens, will give an excellent three (3) 
foot spot at any distance up to 40 feet. This spotlight can- 
not be used to give a five foot spot at 100 feet, because the 
hood is not long enough. In other words, it is impossible to 
get the crater far enough from the lens. 

"The following data will be of assistance to the projection- 
ist, or to the spotlight operator who may wish to obtain a five 
foot spot at various distances. You will note that some 
lenses give the same size spot at widely different distances, 
without altering distance of crater to lens. This is because 
at the outer end of the beam its diameter varies but little over 
a long distance. 

"NOTE. — Before ordering lenses it is of the utmost import- 
ance that you try your lamp and see if you can get the crater 
the required distance from face of lens. If you cannot, then 
you cannot get the results you want unless the length of 
hood be altered so that you can get greater distance between 
crater and lens. 

"To gain a five foot spot at longer distance you must be able 
to get the crater back a greater distance, but it must be con- 
sidered that the further the crater is from the lens the 
greater will be the waste of light — see page 162, hence when 
crater-to-lens distance is increased the amperage must also 
be increased if the brilliancy of the spot is maintained. For 
this reason our long distance spot lamps are equipped with 
a 35, 50, 70 and 100 ampere rheostat." 

IMPORTANT. — It is essential that proper carbons be used. 

If this be not done there will most likely be shadows, rings 
and lack of brilliancy in the spot. It is not sufficient — in fact 
it will not do for the projectionist or spotlight operator to 
place a pair of ordinary ^ths cored carbons in the lamp. 
With direct current, cored projection carbons must be used 
for upper and solid "Silver Tip" carbons for lower. When 
using A. C. use cored white flame carbons. 



To obtain approximately a five (5) 


to stage 

75 feet 
100 feet 

75 feet 
100 feet 
150 feet 

75 feet 
100 feet 
125 feet 
150 feet 

75 feet 
100 feet 
125 feet 
150 feet 

of lens 
5 inches 

5 inches 

6 inches 
6 inches 
6 inches 
6 inches 
6 inches 
6 inches 
6 inches 
8 inches 
8 inches 
8 inches 
8 inches 


length of lens 

9 inches 

9 inches 

12 inches 

12 inches 

12 inches 

13 inches 
13 inches 
13 inches 
13 inches 
13 inches 
13 inches 
13 inches 
17 inches 

foot diameter spot. 
Distance crater 
must be from face 

of lens to obtain 
5 ft. diameter spot 

7 inches 

8*4 inches 
11^4 inches 
11^4 inches 
11^4 inches 
12" for 4' spot 
12" for 4' spot 
12" for 4' spot 
12J4" for 4' spot 
\2y 2 inches 
\2 l / 2 inches 
12^ inches 
\2y 2 inches 






Electric Meters 

THE watt hour meter is the instrument now in general 
use for measuring the electric power consumed. The 
measurement is in watt hours, the meaning of which is 
that a certain number of watts have been used for a certain 
given number of hours, the use of one watt for one hour 
being the unit of measurement. The principle of operation 
of the electric meter is as follows : 

The dial which records the consumption of the power is 
operated by a specially constructed, very small motor, placed 
in series with the current consuming apparatus. The motor 
is so constructed that if it were operated at a pressure of 
one volt for a period of one hour, during all of which time 
one ampere of current flowed, it would record one watt, or, 
in other words, one watt hour. This means that if, for in- 
stance, the motor be run for one hour under a pressure of 
110 volts, with 10 amperes of current flowing, it would move 
the dial hand just far enough during that hour to record 
110 x 10 = 1,100 watt hours, or 1.1 kilowatt hours, or if the 
pressure be 110 volts and the amperage 100, then during the 
same time the motor would move the dial hands far enough 
to record 110 x 100 = 11,000 watt hours, or 11 kilowatt hours- 
This roughly describes the principle of operation upon 
which the electric meter is based, and in a work of this 
kind that is all that could be expected, because to give you a 
thorough detailed understanding of meters would consume 
a great deal of space, without commensurate benefit. 

TESTING METER.— If it is suspected the meter is wrong 
the instrument may be roughly tested in several ways, one 
of which would be to connect an ammeter into the lines and 
a volt meter across the lines near the meter. Read the 
meter and then burn a number of lamps for a period of 
exactly one hour, during which time the meter must record 
exactly the wattage obtained by multiplying the reading of 
the ammeter by the reading of the volt meter, both of which 
should first be tested to make sure that they are correct. This 
is not intended as a conclusive test, but if after making such 
a test there is a discrepancy between the meter and the 
wattage indicated by the volt meter and ammeter, then you 



should insist upon the power company making a regular test 
of the meter. In making such a test, however, it is im- 
perative that the voltage and the amperage remain abso- 
lutely constant during the entire period of the test. 

READING THE METER.— An electric meter is read pre- 
cisely the same as is the gas meter. First carefully note the 
unit at which the dials are read. On all meters used by the 
Edison Company the figures above or below indicate the 
value of one complete revolution of the pointer, hence one 
division indicates 1/10 of the value of the complete revolu- 
tion of the dial hand. Carefully note the direction of rota- 
tion of the dial hand, as indicated by the figures, the pointers 
moving, of course, from to 1, 2, 3, 4, etc. Each dial will 
read in an opposite direction to its neighbor. 

Counting from right to left on a five-dial register the 
pointers of the first, third and fifth dials of a watt hour 
meter rotate in the direction of the hands of a watch, or to 
the right, while the hands of the second and fourth move 
in the opposite direction, or counter-clockwise. The same 
holds true of the four-dial register. The hands of the first 
and third dials move to the right and the second and fourth 
to the left. The dials must always be read from right to left, 
and the figures set down as read, remembering that until 

No. 1. 

No. 3. 

1000.000 100.000 10,000 


No. 2. 

No. 4. 

Facsimiles of Meter Dials, 

Figure 76C 


the hand has reached a division that division must not be 
counted. For instance : In No. 3, Fig. 76c, the right-hand 
dial has passed 1, but has not yet reached 2, therefore it 
reads 1. Likewise the second or 100 dial hand has passed 2. 
but has not reached 3, hence it reads 2. Taking No. 1, Fig. 
76c for example, it reads as follows : A complete revolution 
of the right-hand dial would be 1,000 watt hours, but the 
pointer has just reached the figure 1, which, being 1/10 
of 1,000 is 100. We therefore put down 100. The next dial 
stands at 1, which, since one division is 1/10 of the total 
of 10,000, equals 1,000. Therefore we set down 1 at the left 
of the 100, and have 1,100. The next dial also stands at 1, 
which being 1/10 of 100,000, is 10,000, so we set down an- 
other 1 at the left of 1,100, and have as a total 11,100. The 
next dial stands at 1, so we set down another 1 to the left, 
and as a result have 111,100. The last dial stands at 1, 
which calls for still another 1 at the left, and we have a final 
reading of 1,111,100 watt-hours. No. 3, Fig. 76c, reads in 
k. w. hours. It is read the same as is No. 1. The right-hand 
dial registers up to 10 k. w. h. The pointer is passed 1, but 
has not yet reached 2, therefore we put down 1, that being 
1/10 of the total of 10. The pointer of the next dial has 
passed the 2, but has not yet reached 3, therefore we pu< 
down a 2 to the left of 1. The pointer of the third dial 
reads 1, and that of the fourth 9, therefore we have a total 
reading of 9,121 k. w. h. In No. 2, Fig. 76c, the pointer stands 
at 9, which would mean 900 watts. The next three dials 
stand at 0, therefore we precede 900 with three 000's, thus 
000,900. The pointer of the last, or 10,000,000 dial, stands at 
1, so that the reading would be 1,000,900 watt-hours. The 
reading of No. 4 would be 1,097 kilowatt hours. 

CAUTION. — Some meters read as per their dial indication. 
Other meters are not direct reading, but require that the 
actual reading shown by the dials be multiplied by a con- 
stant in order to obtain the correct reading. This is for the 
purpose of keeping meters of various capacities at fairly 
uniform size. If the constant were not used meters of larger 
capacity would be of greater dimensions than those of small 
capacity. If the register face bears the words "multiply 
by 3," or any other number, then you must multiply the 
actual reading as indicated by the various dial faces accord- 

The theatre manager or the projectionist should always 
read the meter when the company man reads it. 


The Film 

THE film is a strip of celluloid 1% inches wide by from 
5^2 to 6 one thousandths of an inch in thickness. In 
the process of making the celluloid is originally in 
strips 41 inches wide by two thousand feet long. These wide 
strips are passed through a machine which spreads upon one 
side a coating (negative or positive, according to the use to 
which the stock being treated is to be put) of photographic 
emulsion. The emulsion is a part of the thickness of the 
film as above given. 

Having received its emulsion coating, the film is passed 
through another machine which splits it into ribbons VA 
inches wide, and these ribbons become the film stock which is 
purchased by the photoplay producer. 

The negative stock is first perforated, then it is placed in a 
camera having an intermittent movement, a revolving shutter 
and a lens, the whole mechanism being very similar in its action 
to that of the motion picture projector, except that the 
mechanism and film are enclosed in a light tight box, or casing. 
Each Y\ of an inch of the negative is successively exposed to 
the light by the camera mechanism, and what is nothing more 
nor less than a "snapshot" photograph is impressed thereon. 
The exposures are supposed to be at the rate of 16 per second, 
but in actual practice camera speed varies considerably, run- 
ning as high as 80 feet of film to the minute in some instances. 

After exposure the negative is removed from the camera, 
developed, fixed and dried by much the same chemical process 
as is any ordinary Kodak negative, though the mechanical 
methods necessarily differ widely from the Kodak process, 
since the negative film will be anywhere from ten to 300 feet 
in length. 

The negative is then projected to a screen, so that the 
director may check up his work, make the scene over again 
if necessary, or cut out any undesirable portions. When the 
negative is finally in acceptable form, it is placed in a printing 
machine in contact with a strip of positive film 
(positive and negative film are precisely the same, except 
that a different grade or kind of photographic emulsion is 


used) and by means of another intermittent movement and 
revolving shutter, but without the lens this time, it is ex- 
posed to artificial light of fixed, known power, each picture 
being exposed for the small fraction of a second. The posi- 
tive film is then developed, fixed and dried, after which it is 
sent to the assembling room, where the various scenes con- 
stituting a complete photoplay are arranged in sequence, 
joined together, the titles and sub-titles inserted, and it 
finally becomes the "reel of film" with which we are all 

The foregoing, of course, only very roughly describes the 
various processes through which the film passes in the 
course of its making and the making of the play. To de- 
scribe all the processes in detail would require a book of 
goodly size in itself. 

The perforating usually is done by the producer, though 
perforated stock may be purchased from the film stock 
maker. There are 64 perforations to the foot on either side, 
or 4 on each side to each picture. Of late years film per- 
foration has been brought up to a state of almost absolute 
mechanical perfection. It is one of the processes which 
must be done with great accuracy, else there will be un- 
steadiness of the picture on the screen. 

THICKNESS OF FILM STOCK.— It is important that 
film stock be of unvarying, standard thickness, since thin 
stock has a decided tendency to produce unsteadiness of the 
picture on the screen, besides being unduly weak and short 

MEASUREMENTS.— At present (1927) there are several 
shapes of sprocket hole; also the dimensions of various 
sprocket holes vary. The Society of Motion Picture Engi- 
neers has adopted as standard the dimensions shown in 
Fig. 77. 

We quote the following from a paper read by Donald 
Bell before the Society of Motion Picture Engineers at its 
New York City meeting in 1916. 

"It is accepted as settled tact that the maximum shrinkage 
of motion picture film is .0937 of an inch per foot. Painstaking 
experiment warranted the conclusion that a gauge length of 
11.968 inches for 64 holes would insure that accuracy of 
perforation necesary to perfect results, and at the same 
time make due allowance for the shrinkage of film. The 



following computation shows why we have adopted 11.968 
inches instead of 12 inches as the standard for a perforation 
gauge measuring 64 holes : 

"Assuming the outside diameter of the sprocket of all 
standard projectors to be .9375 (15/16) of an inch, then the 
circumference would be 2.94525 inches. As a standard motion 
picture film has an average thickness of .0065 of an inch, the 
pitch diameter of the sprocket will be found to be .9375 of 
an inch, plus .0065 of an inch, or a total of .944 of an inch. 

"Pitch circumference is 3.1416 x .944 = 2.965704 inches. Cir- 
cular pitch equals 2.965704 -*- 16 (number of teeth on a 
sprocket) = .1853+ of an inch. 

"The standard perforating gauge being 11.968 inches for 64 

holes, and the 

078 -I r 



maximum a 1 - 
lowance for 
shrinkage be- 
ing .09375 
(3/32) of an 

for 64 

Figure 77, 


inches minus 

.09375 of an 

inch, or 11.8743 

inches, is the 

average length 

o f shrunken 

film measuring 

6 4 sprocket 

holes. The 

pitch of the 

film, or length per hole, is 11.8743 divided by 64, or .18553 of an 

inch. Pitch of sprocket .1853 of an inch. Pitch of film .18855 

of an inch." 

Pitch means distance from center to center. 
The dimensions shown in Fig. 77 represent good practice, 
and it is to be hoped that in the near future all film dimen- 
sions, including the sprocket holes, will be thoroughly 
standardized along these lines. 

DAMAGE TO FILM.— When film is new, pliable and de- 
cidedly tough, its celluloid base is at that time least sus- 
ceptible to damage, but, on the other hand, that is the time 
the photographic emulsion is most easily damaged. By un- 
intelligent handling of the film, lack of care in the adjust- 



ment of the projector, improper lining of the two elements 
of the rewinder, too-rapid speed of rewinding and the im- 
proper storing, great damage is caused, which mounts into 
many thousands of dollars a day, and this unnecessary dam- 
age must, in the very nature of things, be added to the "over- 
head" expense of the industry, and finally be paid for in the 
form of increased film rentals. 

A very large percentage of the scratches in the photogra- 
phic emulsion of film, which same fill up with dirt and 
form the "rain" with which we are all familar, is caused by 
improper rewinding, particularly the process known as 
"pulling down." 

This latter consists of holding one reel stationary while 
the other is revolved to tighten the film roll, which operation 
causes all the various layers of film to slip upon each other 
under much friction, and since there are always more or less 
particles of dust and dirt adhering to the film, scratches are 
the inevitable result. 

Injury to sprocket holes is, for the most part, due to 
undercut or hooked sprocket teeth (see general instruc- 
tion No. 7) to too much pressure by the tension shoes 


j A 








Model Cement Bottle 

Stock size Gooch Funnel from any drug store. Illustration makes rest 



of the projector (see general instruction No. 9, Volume II 
and to excessive takeup tension. 

As a general proposition we believe that all projectionists 
and some operators are at least reasonably careful in hand- 
ling and repairing film. In many theatres, however, re- 
winding, threading the projectors and repairing film is made 
the duty of a more or less irresponsible usher or reel boy, 
whose main idea is to get the job finished in the least pos- 
sible time. These boys do not understand the damage done 
by careless work; also undoubtedly many of them do not 
care. If a splice is to be made, their one and only idea is 
to get the film ends stuck together. In their view the quick- 
est way is the best way, regardless of after-results. Badly 
matched splices, misframes, splices without the emulsion 
scraped off or only partly scraped off are the regular thing 
where an usher or reel boy does the repairing. It is no un- 
common thing where this sort of irresponsible help is placed 
in charge of repairing film for an exchange to receive film 
back "spliced" with a nail or a pin. Even when the pro- 
jectionist does the rewinding and repairing he is, in all too 
many cases, expected to do it while projecting a picture, 
hence must neglect either one thing or the other. 

In the majority of cases the real underlying fault is in the 
failure of the theatre management to employ sufficient com- 
petent help in the projection room. Film repairing should, 
under no circumstances, be done by any other than a thor- 
oughly competent, responsible projectionist or a regularly 
employed projectionist apprentice. 

Injury to film in passing through a modern motion picture 
projector is invariably due either to the bad condition of the 
film itself, to the false economy of a theatre management 
which refuses necessary repairs to the projector, or to the 
lack of knowledge, carelessness or laziness of the pro- 
jectionist himself, which results in improper tension adjust- 
ment, hooked sprocket teeth, etc. 

Film exchange managers seem, in all too many cases, not 
to realize that the sending out of film in poor condition not 
only is an outrage against the producer, against the pro- 
jectionist who must use it, against the theatre management 
which is paying for films in good repair, but also against the 
audience which pays money to see at least a reasonably per- 
fect performance. The average exchange manager does not 
seem to understand that sending out film in poor condition 
is a direct invitation to more and greater damage, since a 


loose splice is likely to catch on a sprocket idler and split 
anywhere from one to four feet of film before the projector 
can be stopped, especially in houses where the projectionist 
is obliged to rewind and do other chores while his projectors 
are running the show. 

Splices in which sprocket holes are not properly matched 
are likely to clamp the sprocket teeth, thus causing a jump 
in the picture and per- 
haps the loss of a loop, 
or they may grip the 
teeth of the sprocket and 
wrap around it, particu- 
larly if the sprocket 
teeth be under cut or 
somewhat hooked. Split 
sprocket holes will oft- 
times catch on a sprocket 
idler, and a section of the 
edge of the film will split 
off, even if nothing worse 


— Much damage is done 
to first run film by means 
of emulsion deposit (see 
general instruction No. 
10, Volume II). This 
trouble is greatly aggra- 
vated if the projectionist 
carries too tight a gate 

In fact there are num- 
berless ways in which 
film may receive damage. 
It is a fragile product, Figure 77B 

and a product which 

must be in absolutely perfect condition if there is to be a 
perfect picture on the screen. 

FILM WAXER— When using first run films upon which 
the emulsion is soft there is always the inclination of emul- 
sion to rub off and deposit on the tension shoes or springs. 
The best method of preventing this is to place a small 
amount of suitable wax on the sprocket hole tracks. 



It is quite possible to make a home-made waxer which 
will work very well. Such a device is illustrated in Fig. 77C. 
The construction of the device is made clear in the illustration. 



EN3J V/^w 

Figure 77C 

Either ordinary tallow candles or paraffine wax candles may be 
used, or cylinders of wax may be made by using a tin mould of 
suitable diameter, open at both ends. Pour this full of melted 

paraffine wax, which 
may be had at any drug 
store and most grocery 
stores. Let it cool and 
then slightly heat the 
receptacle whereupon 
the wax cylinder may 
be pushed out. 

FILM, i. e., making 
splices in it, is a matter 
deserving of very much 
more consideration 

Figure 77D than . lt a PP arentl y has 

Humidor can for film. May be used — . - 1# 

for keeping film pliable, or for re- Jroorly made splices 

saturating old, dry film with moisture, are a source of great 

though if used for the latter purpose film - _. 

must be wound very loosely. and unending annoy- 



ance to all concerned, as well as the source of literally 

enormous damage to film. 

It has even been claimed by competent projectionists that 
badly made splices are a greater source of damage to film 
than all other things combined. And there is good ground 
for their claim, too, because a poorly made splice may cause 
damage in many ways, not the least of which is to start a 
film split, which may continue for several feet before the 
projector (and the show) can be stopped. 

One source of very great annoyance is the imperfect 

Figure 77E 

A and C are broken sprocket holes improperly notched, C being very 
bad, indeed. B is properly notched except that the notch spreads too 
wide and weakens adjoining holes. D and E are examples of atrociously 
made splices. D is half an inch wide and not properly scraped. F is 
crooked and E is an example of slovenly work in scraping the emulsion 
off. G is a mis-frame and is otherwise improperly made. 

matching of sprocket holes so common where splices are 
made by hand. Imperfectly matched sprocket holes are apt 
to cause the following troubles: (a) Picture jumps as the 
splice goes through because the sprocket holes are too small 
to allow sprocket teeth to set properly, hence film is lifted 
away from the sprocket, (b) A too-small sprocket hole lock- 



ing on sprocket tooth and pulling film around under sprocket, 
(c) Film running off sprocket, (d) Intermittent sprocket 
teeth "climbing" one or more holes, thus shortening or 
"losing" one of the loops, and throwing the picture out of 
frame on the screen, (e) Side movement of picture due to 
crookedness of film as a whole, (f) Takeup, pulling film over 
lower sprocket, thus shortening or losing the lower loop. 

All this is apt to occur, even though the sprocket holes be 
perfectly matched on one side, if they be imperfectly 

Figure 77F 

A is a splice which may cause sprocket holes to clamp sprocket teeth, 
with consequent jump of picture on screen, or even the pulling of the 
film around sprocket. It certainly will cause side movement of picture 
on screen. At B is a broken sprocket hole which has been improperly 
notched, proper shape of notch being shown at D or F, though notches D 
and F are correct as to shape only. They should not have been made 
either in or so near a splice. As the injuries to the film now are all 
that portion ,from A to D should be cut out and a new splice made. At 
G a splice is necessary because the injury includes three successive 
sprocket holes. 

matched on the other side, because in that event the hole or 
holes on one side will be small, and the film, as a whole, will 
be crooked at that point. 

You will therefore see the great importance of matching 


the sprocket holes perfectly, which in practice means using 
either a splicer or some sort of metal teeth for a guide. 

In many projection rooms the practice is to make splices 
with the unaided fingers. Film cement welds, more than it 
glues the film together, hence evenly applied pressure, of 
considerable amount, is necessary to the making of a per- 
fect joint, and while it is quite possible to apply sufficient 
pressure with the fingers, it certainly cannot be applied 
evenly, with the almost inevitable result that even though 
the finger-made splice be strong in part of its width, it will 
be weak in another part. 

In Fig. 78 you sec a compact and very effective film 
splicer. It is made by a New York City manufacturer. 
We have had this 
device tested and have 
tested it personal- 
ly. It is excellent 
and well worth its 
price, which latter is 
quite reasonable. We 
advise the installation 
of this device or a 
similar one in all pro- \ 

jection rooms, and 
that the making of 
hand-made splices be Figure 78. 

absolutely prohibited. 

WIDTH OF SPLICE.— A too-narrow splice is apt to be 
weak, and a too-wide one objectionably stiff. Provided a 
good film splicer be used, there is no necessity for a splice 
of greater width than .125 (%) of an inch. Such a splice will 
be amply strong and at the same time sufficiently flexible to 
go through the projector without any indication of its pres- 
ence in the film showing on the screen. 

There has been a machine used by some producers which 
makes a splice about 1/32 of an inch wide. There is no 
practical advantage in the use of such a splice in positive 
film, and these very narrow splices have been an unending 
source of annoyance to the projectionists. The maker of the 
machine places the blame on those handling it, but that is 
no excuse, because a machine which does not function well 
in the hands of the employees who must be depended on to 
handle it is not a good machine, notwithstanding the fact 
that it would produce good results if expertly operated. 



Anyhow, as we have said, there is no advantage in such a 
narrow splice in positive film. 

There should always be one full sprocket hole in the stub 
end, as at A, Fig. 79, and stub end A should never exceed 
.125 (%) of an inch in width, unless it be necessary to 
slightly exceed that width in order to avoid cutting into the 
sprocket hole. 

MAKING THE SPLICE.— Cut the film ends as per Fig. 79, 
end B being trimmed exactly on the dividing line (frame 
line) between two pictures, the other end with a stub end 
(A Fig. 79) extending .125 (y$) of an inch beyond the frame 
line indicated by dotted line. 

It is of the utmost importance that every particle of emul- 
sion be scraped off stub end A; also that the back, or cellu- 
loid side of end B be scraped lightly in order to roughen the 
celluloid and remove all dirt and grease. 



^Qver/app/ng of Pi7m- 

Relative Position of Pilots and Perforations 
in New Pi I m in Old Fitm 

Figure 78A. 

Some prefer a very sharp knife blade and some a safety 
razor blade fixed in a convenient holder, to scrape with. What 
is used does not matter, provided a thorough job be done, 
without removing any appreciable portion of the celluloid 
itself, since that would weaken the film stock. 

OF IMMENSE IMPORTANCE.— It must be understood 
and remembered that the emulsion covers the entire film, 



from outside to outside, and that, except for special cements, 
film cement will not adhere to emulsion, and will not pene- 
trate it and weld the celluloid beneath — for film cement does 
not merely stick the surfaces of the film together, but 
actually welds them, if the cement be a good one. And this 
is right where the greatest sin is committed in making 


Figure 79. 

Either from laziness, carelessness or lack of time to do the 
job right, or because they fear to break the edge of the film 
at the sprocket hole, many do not scrape the emulsion off 
thoroughly, or even do not scrape it off at all around the 
ON THE SPLICE WILL COME. The inevitable result is 
that such splices either never are cemented together at their 
edges, or else very quickly come loose at the sprocket holes, 
whereupon there is, sooner or later (usually sooner), trouble. 

SCRAPE TO A STRAIGHT LINE.— Stub end A, Fig. 79, 
should be scraped to a straight line at the frame line, as per 
dotted line, and the use of a straight edge is imperative to 
this end. Too much trouble? Well, if you think so, then 
you ought not to be allowed to handle film at all. Careless- 
ness in this respect means flashes of white light on the 

Apply cement to scraped surface A by flowing it 
on from tip of brush. Never apply cement with paint- 
ing movement of brush. Just flow it on from brush 
tip with one stroke. This last is important. The 


actual methods of placing the ends together will, of course, 
vary according to whether or not a splicer is used, but in any 
event one must work fast, match the sprocket holes of the 
two ends perfectly, and then apply tolerably heavy and 
evenly distributed pressure for say five seconds. 

Every cement bottle should have a small brush, the handle 
of which is attached to the under side of the cork, or else 
thrust through it. When you buy cement, accept none with- 
out the brush, unless you already have an empty bottle thus 

FILM CEMENTS.— The Eastman Kodak Company has on 
the market a film cement which is, so they assert, made from 
tested chemicals, which means that every lot will be exactly 
the same. This cement is equally good for either inflammable 
or non-inflammable film. Certainly the Eastman company, 
being the largest manufacturers of film in the world, ought 
to know what is required in a film cement, and if tested chem- 
icals only are used, that should be sufficient guarantee to us 
all that the cement is good. 

FORMULAS. — In presenting these formulas please under- 
stand we do NOT vouch for their excellence. They are all 
good when made from proper chemicals, but experience has 
amply proven the fact that when a certain formula for film 
cement is made up at different times from chemicals purchased 
from various local drug stores, there not infrequently is a 
wide variation in results. We therefore present cement form- 
ulas subject to that notation. 

ORDINARY.— For non-inflammable stock, l A pound of 
acetic ether, 54 pound of acetone merch, in which dissolve 
6 feet of non-inflammable film from which the emulsion has 
been removed. (See Removing Emulsion, Page 290.) 

For inflammable film, a piece- of the film 3 inches long 
dissolved in 1 ounce of acetic ether is a satisfactory cement, 
but it will not work on N. I. (non-inflammable) stock. In 
dissolving the film, in either case, first remove the emulsion 
and then cut the film into fine strips. 

ACETONE CEMENT.— Four ounces of acetone; V* ounce 
ether; 6 inches old film, from which remove the emulsion 
and cut into strips. 


ANOTHER FORMULA.— Equal part? of amyl acetate and 
acetone. Will not turn white on film, and will not dissolve 
the film as ether will. Works on all kinds of stock. Best 
used with an all steel 3 flap film mender. Can be used 
by those making patches by hand if worked properly. Scrape 
film, use small camel hair brush; keep bottle tightly corked 
when not in use. 

STILL ANOTHER.— One ounce collodion; 1 ounce banana 
oil or bronzing liquid; Vi ounce ether. For Pathe hand 
colored films, J4 acetone and V2 ether. 

N. I. CEMENT.— For non-inflammable film add 1 part 
glacial acetic acid to 4 parts of flexible collodion to any of 
the film cements. It is satisfactory for either N. I. or 
regular film 

FILM REEL CONSTRUCTION.— For many years the weak, 
flimsy construction of reels worked huge damage to the films 
the reels were supposed to protect. The earlier practice wa§ 
to use a wooden hub \ l / 2 inches in diameter, to which the thin, 
weak sheet metal sides were attached by means of three or 
four ordinary wood screws on either side. The hub itself was 
often made of rather poor wood, its diameter was so small 
that it furnished little support to the weak metal sides, with 
result that within a very short time the reels would have bent, 
crimped sides, and be in more or less wretched condition — 
condition which actually offered more of damage than pro- 
tection to the films. 

Another fault commonly found was due to the punching out 
of the sheet metal sides. There was a decided tendency, es- 
pecially after the dies became worn, for the metal to drag 
through, leaving a fringe of sharp metal points around the 
openings in the sides of the reel, and perhaps around its 
edges too. 

In the rush of production, and the insane demand for 
cheapness in first cost, these rough edges were not ground 
off, and the points cut and tore the film edges sadly. 

Another common source of damage to films by reason 
of the use of such flimsy reels, was that their bent sides 
would often rub on the sides of the projector upper mag- 
azine, thus acting as a more or less powerful brake, against 
which the film must pull. This braking action was often 
so bad, particularly toward the end of the reel, that the 
sprocket holes would be cracked, or the film actually pulled 
in two. Also the sides would often be so bent inward 


that they would pinch the film at each revolution of the 
reel, both on the projector and the rewinder. 


The exchanges sought the cheapest possible cheapness in 
these respects, entirely ignoring the enormous cost in damage 
to film. 

Of late years, however, this absurd situation has begun 
to remedy itself. Exchange men are coming to an under- 
standing that a "saving" of one dollar which causes damage 
of far more than that amount is at least questionable practice, 
and the tendency has been to make better, heavier reel con- 

Room Reels," Page 322. 

For general use we heartily recommend the steel wire weld 
reels now being made. They have had a thorough try-out in 
practical use. They are strong and offer a minimum possibil- 
ity of damage to the films. 

In considering this recommendation exhibitors should re- 
member that all damage to film which tends to shorten its 
useful life must inevitably come back to the exhibitor in 
the form of increased film rental, hence the less damage done 
to the films while in their theatre the less will be the gen- 
eral overhead expense to be charged back in this way. 

SIZE OF REELS.— The present trend is to use 2,000 foot 
reels in the process of projection, though for the most part 
film is still snipped on 1,000 foot reels. 

But whatever the capacity of the reel, one thing is im- 
portant, viz. : Its sides should always extend over the film 
roll by at least 54 and preferably y 2 an inch for protection. 
The evil of the overloaded reel is three-fold, (a) That 
portion of the film outside, or above the sides of 
the reel, is absolutely unprotected, hence liable to injury 
in many ways other than the likelihood of its slipping 
off, to the exasperation of the projectionist and the 


possible delay of the show while it is being wound on again, 
to say nothing of probable damage through contact with a 
more or less dirty, dusty floor, (b) (Very serious indeed) 
the increased temptation to "pull down," and pull down good 
and hard, too, in order to get as much of the film inside the 
reel sides as possible, (c) The fact that the film may rub 
against the magazine, thus scratching it and possibly inter- 
fering seriously with the operation of the takeup, inci- 
dentally requiring an excessively tight takeup tension, which 
is worse than bad, and a prolific source of damage to the first 
part of the film through scratching and the tendency to pull 
the film over the teeth of the lower sprocket, thus injuring 
the sprocket holes, scratching the film and losing the 
lower loop. 

been asked many times how to figure what number of feet a 
reel of given diameter, with a hub of given diameter, will hold. 

This is a question which cannot be answered exactly, be- 
cause it* will depend upon how tightly the film is wound. It 
is possible to figure it, though we cannot recommend the 
process for accurate results. 

First find average length of film layers, which is done by 
adding together the circumference of the reel hub and the 
outside circumference of the film roll, when the reel is full, 
and dividing the result by two. The result will be the average 
length, in inches, of all layers of film. 

Next subtract half the diameter of the reel hub from half 
the diameter of the film roll. The result will be the number 
of inches of film, or the "depth" of the film roll from outside 
diameter to hub. 

Next you may either count the number of layers of film 
in one inch, or you may divide 1,000 by 6 (six thonsandths of 
an inch being the thickness of film and emulsion), which will 
give you the number of layers of film per inch, provided the 
film be very tightly wound. Counting is best, though, and 
even that will be unreliable, because of variation in tightness 
of winding. 

You now multiply the number of layers of film per inch 
by the number of inches of depth in the film roll, and multiply 
that result by the average length of the layers. Divide this 
by twelve, to reduce to feet, and the final result will be the 
number of feet of film on the reel, or which a reel will hold, as 
closely as it can be figured. 


LEADER AND TAIL-PIECE.— For several reasons it is 
essential that there be a "leader" and "tail-piece" at the ends 
of every reel of film, including the multiple reel feature. 
The leader not only serves to protect the title from dam- 
age, but it enables the projectionist to thread his projector 
with one of the first frames of the main title over the aper- 
ture, whereas otherwise by the time the projector was 
threaded much of the main title would be past the aperture. 
Then, too, in threading into the takeup it is frequently de- 
sirable, if not necessary, to fold an inch or so of the film 
over on itself. By so doing it is made stiffer and more easily 
thrust under the reel spring, also it is more certain to "stay 
put." This means that the film will soon break off where 
it is folded, which causes a gradual wasting away of the 
main title, if there be no leader, and soon there will either 
not be sufficient title, or a new one will have to be provided 
for. If, however, there is a leader, then there is no wasting 
of the main title, and leader costs very much less than title. 
Another reason why leader should be used is that at the end 
of the process of rewinding the film often slaps around any- 
where from one to a dozen times before the reel is stopped, 
and if there be no leader to receive the brunt of this rough 
treatment the title itself is injured. Leader should be at 
least 36 inches long, and 5 feet is very much better. Old 
film may be used for leaders, but the better plan is to use 
film upon which no photograph has been impressed, but 
which has been exposed in the printing and developed quite 
dense. This has the advantage of allowing the man who is 
too lazy to thread in frame to frame up before the main 
title comes on. 

As a matter of fact no competent projectionist who has 
pride in his work will even think of threading out of frame, 
but there are a considerable number of operators, or mis- 
called projectionists, who still persist in sloppy methods. 
They have no pride in their work, and no right place in a 
projection room. Instead of being paid for their work the 
motion picture industry would do better to, if necessary, pay 
them to remain entirely outside of all projection rooms. 

Let us here remark that some projectionists use a stere- 
opticon title where the film title is too short to show. A 
better plan than this is to punch a hole about % of an inch 
in diameter in the center of the dowser, then before starting 
the projector, with the dowser down and the revolving 
shutter of the projector turned until the projection lens is 



open, raise the automatic fire shutter and project the film 
title to the screen. If the hole in the dowser he not too large 
(% inch in diameter ought to do the trick all right) there 
will be absolutely no danger of injury to the film, and at the 
same time the title will appear on the screen plenty plain 
enough to be read by the audience. It is a much better plan 
than using a stereopticon slide. 

Our reason for advising a leader and tail-piece on multiple 
reel features is found in the fact that whereas they will 
slightly inconvenience the projectionist of the large theatre, 
who joins his 1,000 foot reels together in 2,000 foot reels, 
they will be very necessary to the projectionist who projects 
from 1,000 foot reels. 

AN OPAQUE TAIL-PIECE is of great importance, be- 
cause we know of nothing in all the realm of projection so 
thoroughly disillusioning as the flashing of white light on the 
screen at the end of a reel. The careful, competent pro- 
jectionist will stop his projector, or will change over to the 
other projector before the end of the reel comes. This man will 

Figure 79A. 

Graphic illustration of a reel rightly rewound A, which may only be 
done by applying considerable and even brake friction to the reel from 
which the film is being rewound, and a reel improperly rewound, which 
means without the tension supplied by brake action on reel from which 
film is being wound. Film rewound as per reel B is liable to serious 
damage in several ways. It is "pulling down" to tighten films rewound 
as per B which causes nine-tenths of the "rain." 


need no tail-piece, but the sloppy man who either does not 
care or is too lazy to do his work right, and who lets the end 
of the film pass the aperture before changing over to the sec- 
ond projector should have an opaque tail-piece on all his reels, 

Figure 79B. 

At A we see an improperly packed shipping case. Resultant possible 
damage to the films is apparent. At B we see a properly packed 
shipping case. Remarks seem unnecessary. 

so that instead of the screen going white it will go dark, 
provided, of course, he is sufficiently on the job to stop the 
projector while the tail-piece is over the aperture. 

The allowing of the entire film to run through and the 
flashing of the white light on the screen at the end of the 
run is very crude work indeed, so crude that no man de- 
serving the title projectionist would even think of doing it. 
Even the "operator" should be ashamed to do such a stupid 

FILM INSPECTION— The projectionist should, so far as 
is practicable, repair all damage, other than ordinary wear 
he himself inflicts upon film while it is in his possession. 

It is the duty of film exchanges to thoroughly inspect and 
repair all films before they are sent to a theatre. THE 





iirrrtvii • 

ir : ""p & .~% 

i t m "t fi-§Ae i • 8 n 

Figure 80. 

It is a well known fact that many film exchanges make 
only the most superficial inspection of film, and either very 
little or no repairs at all. The underlying cause of this is, 
we believe, an endeavor by film exchanges to get too much 
work out of the film, coupled with a deliberate attempt 
on the part of the exchange to force the projectionist to do 


their film inspection and repair work free of charge, be- 
cause they are unwilling to expend the necessary amount of 
money in the employment of either enough or competent in- 
spectors. In many exchanges we can personally bear testi- 
mony to the fact that "inspection" and repairs consist of a 
man or girl rewinding the film at top speed, stopping only 
when the film is torn clear in two. We might incidentally 
add that these "inspectors" often used crooked reels and re- 
winders which are badly out of line, under which condition 
by rapid rewinding of the film they actually do more damage 
to the film than their "repairs" amount to. 

The ordinary exchange inspection does not detect any- 
thing except the very worst faults, such as long stretches 
of ripped sprocket holes, a patch loose half way across the 
film or the film torn entirely in two. Minor faults cannot 
possibly be detected by the whirlwind inspection process in 
vogue in very many exchanges. 

We are well aware that the question of inspection and 
repair presents a problem of several angles, and one which 
is not at all easy to adjust. However, the statement that 
there is absolutely no excuse whatsoever for the utterly 
miserable condition in which many films are received by 
the projectionist cannot be successfully contradicted. 

We are heartily in favor of projectionists demanding 
overtime for inspecting and repairing film when they are 
received in bad condition. We are unable to understand by 
what process of reasoning either the exhibitor or exchange 
justifies his demand that the projectionist do* the work 
without remuneration. 

FILMS ON A CIRCUIT.-— Where films are used on circuit 
it should be a point of honor with each projectionist to send 
the films away in as good condition as they are received. 
DON'T leave it to your brother projectionist who gets them 
next to repair the damage YOU have done. 

FILM NOTCHING PLIERS.— For a long time there has 
been on the market a cutting plier with which broken 
sprocket holes may be notched as per Fig. 80. 

This tool should be in the hands of every exchange in- 
spector and projectionist. It is the invention of A. Jay 
Smith, Cleveland, Ohio. 

WHERE TO KEEP FILMS.— Film should be kept near 
the floor of the projection room, since near the ceiling it is 
much warmer. It should be kept in a metal box having com- 


partments for each reel, and 6ne compartment below to hold 
a wet sponge or water. The film should be treated with a 
little glycerine once in a while, but this is only accomplished 
by having the film in actual contact with the liquid, as per 
directions further on. The glycerine is for the purpose of 
keeping the film soft and pliable, which it does by reason of 
the fact that it has the property of rapidly absorbing and 
retaining moisture. 

Should water, by any accident, be spilled over a reel of 
film, or it even be dropped in a pail of water, it may be s^ved 
from damage if unrolled very quickly, not allowing the 
emulsion, which will be quickly softened, to touch anything. 
But the unrolling must be done very quickly or the emulsion 
will stick to the back of the film and pull off. This does not 
apply to colored or tinted film, though even these may some- 
times be saved by very prompt action. The author once 
rescued a first-run film from destruction thus: He happened 
to be in the projection room after the show had closed for 
the night. In taking the last reel from the magazine it slip- 
ped from the projectionist's hands and landed in a pail of 
water, being practically submerged. He grabbed the reel, 
ran down stairs, handed the end to an usher, ran to the front 
end of the theatre, looped the film over a chairback, and ran 
back and forth until the whole film lay across the back of 
the seats. The emulsion became very soft in places, but next 
morning it was found that a total of less than five feet was 
damaged. The exchange men never knew of the occurrence 
until more than a month after, when they were told of it. 

MOISTENING DRY FILM.— Traveling exhibitors often 
find that a film which has been a long time in use has be- 
come very dry and brittle. It may be remoistened and ren- 
dered pliable by unwinding into a large metal can, in the 
bottom of which water has been placed, with a wire screen 
over it to keep the film from contact therewith. Cover 
tightly, set in a moderately warm place until the film is soft 
and pliable. Watch closely, however, since if made too 
moist the emulsion will stick to the back of the film when 
it is rewound. 

It is even possible to give a film a glycerine bath, as fol- 
lows : In a shallow pan a few inches wide by 6 feet long 
place a solution of 30 parts of clear water to one part of 
glycerine. Make a drum of slats about six feet in diameter 
by about six feet long (for one thousand feet of film), and 
by revolving the drum draw the film very slowly through 


the liquid, winding on the drum with the emulsion side out. 
After the film is all on the drum, revolve it rapidly to throw 
off the surplus liquid, then continue to revolve the drum 
slowly until the film is dry. It should not be used for two 
or three days. Perform this operation in a room entirely 
free from dust, or you may seriously injure your film. 

Due to lack of proper exchange inspection it is usually 
necessary to inspect the films at the theatre before 
they are run. To do this place the reel on rewinder, and 
rewind it very slowly, holding the edges between the thumb 
and forefinger with pressure enough to cup it slightly. By 
so doing you instantly detect all stiff or loose patches. Cut 
out the stiff ones and remake. Cement all loose patches and 
notch all split sprocket holes. If more than two sprocket 
holes are missing on one side — that is, in succession, of 
course — cut the film and make a splice. Remember, my friend, 
an ounce of prevention is worth a pound of cure. Managers, 
however, should not expect projectionists to inspect films for 
nothing. Such work is no part of his duty and should by ail 
means be paid for, aside from the projectionists regular 

ing the film in warm water, to which ordinary washing soda 
has been added. Put in large double handful of soda to the 
bucket of water. Wash the film afterward in clean, warm 

CLEANING FILM is a legitimate function of the film ex- 
change. Film gradually accumulates more or less dirt and 
oil, all of which is highly injurious to the screen result. 
There are several cleaners on the market, designed for use 
in the projection room. They for the most part consist of an 
arrangement for holding pads of canton flannel or other 
material between which the film is pulled in the process of 
rewinding. These devices remove considerable dust and 
dirt, and at least some of the oil, but have not proven very 
popular, one reason being that there is always the possibility 
of damaging the film badly by some foreign substance stick- 
ing on the pads and scratching the emulsion in a straight 
line all the way through the film. 

Alcohol will remove dirt and will not injure the emulsion, 
but it is likely to cause the film to curl very badly, hence it 
is not to be recommended for the cleaning of film. 

The Research Laboratories of the Eastman Kodak Com- 
pany is authority for the assertion that film may be cleaned 


with commercially pure tetrachloride without damage, pro- 
vided the same be allowed to thoroughly evaporate before 
rewinding the film. To do this it is necessary to wind the 
film spirally on a drying drum, which must be about six feet 
in diameter by six or seven feet long for a thousand feet of 
film. The drum should be revolved until the film is thoroughly 

WARNING — If the film be cleaned by being pulled through 
a cloth moistened with tetrachloride, and be immediately 
rewound, sufficient of the fluid may, and probably will remain 
to attack and seriously injure the film image by bleaching it 

Film may be cleaned with gasoline, benzine, toluene or 
zylene, but all these are inflammable. 

The Eastman Company says : 'The solvent tetrachlorethy- 
lene, made and sold by the Dow Chemical Company, Midland, 
Michigan, is non-inflammable and can be recommended for 
film cleaning. This substance does not attack the film, and is 
sufficiently non-volatile to remain for a short time before 
evaporating, and so has a chance to dissolve out the grease 
from the film before it is wiped off." 

After using this solvent, however, the Eastman Company 
recommends that, as a precaution, the film be wound on a 
drying drum, as above described. 

There is at least one firm in New York City which makes 
a business of cleaning film. Its process is quite thorough. 
The projectionist who wishes to use the cotton pad method 
can easily construct a device to hold two canton flannel 
pads, each about six inches long, together under some pres- 
sure, between which the film may be drawn in the process of 
rewinding. We do not especially recommend it, but it can be 
done. Th*. pad should consist of at least four or five thick- 
nesses of cloth — the more the better. It is even possible for 
the projectionist to remove considerable dirt and oil by pull- 
ing the film between absorbent cotton cloths held in his 
hands in the process of rewinding. As we said in the first 
place, however, the cleaning of film is a legitimate function 
of the exchange, and we recommend that the projectionist 
confine his efforts to careful handling of the film, to the end 
that no oil and as little dirt as possible be accumulated 
thereon while it is in his possession. 

ing film leaves a sticky, brown, gummy substance on metal. 
This may be easily dissolved and removed by washing the 



metal with ordinary peroxide of hydrogen, which may be 
had at any ten cent store, or at any drug store. 

MEASURING FILM.— All standard professional projec- 
tors made in the United States, or, so far as we know, made 
anywhere in the Americas, pass exactly one foot of film to 
each turn of the crank shaft. The number of feet of film in 
a reel may therefore be measured merely by running it 
through one of these projectors and counting the number of 
turns of the crank shaft, which will be equal to the number 
of feet of film passing through the projector. Thus, if while 
running a given subject the crank shaft of the projector 
revolves 980 times, there are just 980 feet of film in that 

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B:&^ : ^i ; 1 

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Figure 80-A. 

There are also several film measuring devices on the mar- 
ket, which may be had of any dealer in motion picture 

The way these devices operate is illustrated in Fig. 80-A. 
A very good makeshift film measurer may be had by discon- 
necting the intermittent of an old standard projector, using 
only the upper sprocket. One turn of the crank is then 
equal to one foot of film. 


The Projection Room 

THE projection room may properly be described as the 
heart of the motion picture theatre, since from it 
comes at least the major portion, and in some cases 
all of the entertainment provided by the theatre. 

In the beginning of the industry the practice was to house 
the projector in a more or less flimsy enclosure, of the small- 
est possible dimensions, unventilated and located anywhere 
space could be found which had no possible value for any 
other purpose. 

Of late, however, thanks at least in some measure to the 
work of the handbook and the projection department of 
Moving Picture World, exhibitors are beginning to under- 
stand something of the importance of a well constructed, 
commodious, well ventilated projection room; also that un- 
less the same be r o located that the projection lens will be 
central with th<; center of the screen, distortion of the 
picture and other evils will inevitably result; see Page 253. 

LOCATION. — As has been explained under "Keystone 
Effect," Page 253, a location of the projection room which 
will produce a heavy angle of projection will not only result 
in distortion of the picture outline, but also of everything 
within the picture itself, and while it is possible to correct 
the outline distortion insofar as has to do with making the 
sides of the picture parallel, the distortion of objects in the 
picture itself can only be remedied by changing the angle of 
projection, which in practice means changing the location of 
the projection room. 

In considering this matter the exhibitor and projectionist 
should understand that it is the angle which counts. This 
means that distance of lens from screen is a big factor in the 
matter, as is shown in Fig. 75, in which the height of center 
of projection lens above center of screen is the same in all 
cases. The 40-foot distance gives a 32 degree angle, the 80- 
foot distance only a 17 degree angle, while if the lens be 
moved back to 120 feet the angle would be only 11 degrees. 
This teaches us that if the projection distance be short, it is 
necessary, if the distortion is to be confined within a given 
permissible limit, that the height of the projection lens above 


the screen center be much less than if the projection distance 
be long. 

The Society of Motion Picture Engineers has set its seal 
of approval on the following: 

"PROJECTION ANGLE.— The maximum permissible 
angle of projection shall not exceed twelve degrees (12°) 
from a perpendicular to the screen surface. " 

While this is somewhat ambiguous, we may, we think, 
accept it as meaning that the maximum permissible angle of 
projection shall not exceed twelve (12) degrees from a hori- 
zontal line passing through the center of the screen, when 
the screen sets perpendicular. 

An angle of 12 degrees amounts, roughly, to 2.55 inches 
to the foot. By "roughly" we mean that is close enough for 
practical purposes, though it may be a very small fraction 
of an inch more or less. From the foregoing we readily see 
that if we 

Multiply the proposed projection distance, in feet, by 2.55 
and divide the result thus obtained by 12, the final result will 
be the height, in feet, the lens may be above the screen cen- 
ter without exceeding a 12 degree angle of projection. 

By subtracting *the distance of the projection lens from 
the floor of the projection room from the result obtained by 
the application of the above rule we shall have the maximum 
permissible height of the projection room floor above the 
screen center. See remarks concerning 12 degree angle. Page 

In considering possible available projection room locations 
the angle of projection, and consequent distortion are, of 
course, of first importance, though this consideration is 
rivaled by another, namely, 

LIMITS OF VIEW.— Select any familiar object, such as, 
for instance, a tree. Carefully observe it, in normal light, at 
a distance of 150 feet. You will of course see the tree as a 
whole quite plainly. You may even see most of its individual 
leaves, though in some places, unless your eyes are above 
the average, they will appear mostly as green masses of 
foliage, with the outline of individual leaves difficult to trace. 
Advance to a one hundred foot distance. You can now per- 
haps distinguish the outline of all the leaves, but the trunk 
most likely will only show as a light or dark object, without 
much detail of the bark being visible. Advance to seventy- 
five feet and see how much more clearly you are able to 


distinguish the individual leaves, and how the detail of the 
bark begins to show. 

Precisely the same thing which was true of the tree is 
true of the picture on the screen as observed by the projec- 
tionist only, due to difficulty of looking through a compara- 
tively small opening in the wall of an all too often well 
lighted room, the detail of the foliage of the actual tree will 
appear much more clearly than will the detail of the image 
of the same thing on a screen, distance being equal. 

It therefore follows that distance of screen from projec- 
tion room is of vital importance, since if the distance be 
too great the projectionist will not have a clear view of the 
detail of the picture, hence will be unable to judge of the 
fine sharpness of focus, and sharpness of focus is of vital 
importance, since it has intimately to do with eye strain. 

Many exhibitors permit the establishing of an abnormally 
long projection distance, and then try to compensate for the 
poor view the projectionist has of his screen by providing 
an opera glass. This latter is, of course, an excellent thing 
to do, even with a short projection distance, because there 
are times when the projectionist will wish to observe the 
screen very closely, but to attempt to compensate for a too- 
long projection distance thus is in the nature of a makeshift, 
and one which is only partly successful, because a projec- 
tionist just simply will not use a glass as often as would be 
necessary for the best possible results. 

It is just plain common sense that the picture should be 
kept in as sharp focus or definition as possible. 

It is also just plain common sense that if the projection 
room is so far from the screen that the projectionist cannot 
see the finer details of his picture clearly and sharply, the 
picture will not be in constant sharp focus, or at least not 
in the sharpest possible focus. 

Oh yes, we grant you the projectionist can use an opera 
glass, but, as we before said, he won't, at least not with 
sufficient regularity to insure 100 per cent, sharpness. 

It therefore follows that for best results (and any other 
than best results will inevitably react to the injury of box 
office receipts) the projection room must not be placed too 
far from the screen. But to determine the maximum per- 
missible limit of distance is a difficult matter. Perhaps it 
may be best disposed of by saying that beyond seventy-five 
feet the view of the picture from the projection room can- 


not possibly be what it should and must be for best results, 
hence the result will inevitably suffer at least to some ex- 
tent, insofar as concerns sharpness of focus. Of course as 
the distance increases the view of the screen from the pro- 
jection room will become less distinct. 

But this element interlocks with another to the extent 
that any distance sufficiently short to require a projection 
lens of very short focal length — say less than 4 inch E-F, 
is highly objectionable, because very short focal length 
lenses do not give sharpness of definition all over the field. 

The whole subject of projection room location is full of 
complications, but it may be set down as fact that, except 
in those few cases where extraordinary sacrifice would have 
to be made to do it, it will be a paying proposition to so 
locate the projection room that the picture height will not 
be increased by more than 5 per cent, through distortion 
due to angle of projection, and the projection distance 
(throw) such that not less than a 4-inch E-F projection 
lens will be required and as little more than seventy-five 
feet from lens to screen as can be accomplished. 

FRONT OF BALCONY LOCATION.— The location which 
promises best results in large theatres is in the body of the 
balcony. This location is a recognized possibility by some 
architects now, and will, we have faith to believe, become 
increasingly popular when theatres are planned in which 
there is to be a balcony, and in which other available loca- 
tions would either give a too steep projection pitch, or else 
a too great distance of projection. 

In Fig. 81 we see the diagrammatic representation of the 
possibility of such a location. The possible objections are: 
(a) Cost of installation. (b) That the balcony will sag 
somewhat under stress of load, (c) That proper ventila- 
tion will be difficult, (d) That in case of fire there would 
be danger of panic by reason of smoke coming out in the 
midst of the audience. 

These objections are, except for the first named, capable 
of being reduced to practically nothing at all. The balcony 
will settle somewhat, yes, but it is a simple matter to con- 
struct a compensating projector table which will take care 
of this and keep the picture automatically centered on the 
screen. See Fig. 81 A. Such a room may be ventilated as 
much as may be desired. It is merely a matter of cost of 
necessary vent ducts, and their installation; also ducts may 
be easily provided which will carry away every particle of 






smoke and gas in case of fire, so that although a portion of 
the audience may, and probably will, be seated literally 
within eighteen inches of the ceiling of the room, they will 
not be aware of anything more than a stoppage of the show 
should a fire occur. It is a simple problem to make such a 
room practically entirely sound proof. In fact, there is no 
valid objection to such a projection room location, except 
the matter of installation cost. It is a lamentable fact that 

both architects 
and exhibitors, 
with some excep- 
tions, seem im- 
bued with the 
idea that projec- 
tion room loca- 
tion is of no par- 
ticular import- 
ance. This error 
is tremendously 
harmful to the in- 
dustry, because it 
makes for inferior 
results on the 
screen, and in- 
ferior results on the screen make for a less pleasing general 
screen result, with consequent lessened patronage. 

THE MAIN FLOOR LOCATION.— The location of the 
projection room on the main floor of the auditorium offers 
no insuperable, or even largely objectionable difficulties, as 
has been amply proven in the west, where many high-class 
theatres have projection rooms thus installed. It is very 
largely a matter of occupying space which might otherwise 
be devoted to high-priced seats. 

The point the exhibitor who objects to the main floor 
location overlooks, is that with a main floor projection 
room location he gets maximum possibility for screen re- 
sults, hence greater drawing power at his box office. 

Take a theatre seating two thousand, for example. As- 
sume it to give three shows a day. It then has 2,000x3= 
6,000 seats to sell each day. Suppose the projection room 
occupies space in which 30 seats might be placed, and that 
those seats, if filled, will sell for fifty cents each, or forty- 
five dollars a day. Mark you well the IF FILLED. How 
many theatres do sell their entire seating capacity for three 
shows? Very few, if any, except in the case of attractions 

Figure 81 «A. 


which have within themselves an extraordinary drawing 
power, hence we may reduce the loss by at least one-third, 
making it thirty dollars. But even that is a very serious 
matter, unless compensated for. 

And right there, Mr. Exhibitor, we ask you to think very 
carefully. You will, we believe, agree that a clear cut, un- 
distorted picture will be more pleasing to your audiences 
than will the heavily distorted .picture which is not in the 
sharpest possible focus (two faults which are the invariable 
accompaniment of the long projection distance and the top 
of the balcony projection room location) and that the more 
pleasing picture, or more pleasing general screen result 
must and will operate to the benefit of the box office. Is it 
not therefore reasonable to suppose that by the sacrifice of 
some of your high-priced orchestra seats (which will only 
be really sacrificed when you have a capacity house) you 
will sell a greater number of seats at times when your 
theatre is not normally full. In other words, while the per- 
fect screen result cannot increase the business of your 
capacity shows, it can and will increase the business at the 
shows which do not do a capacity business. 

The fact is that the perfect screen result upon a main 
floor location will sell more than enough additional seats to 
make up for those lost, while the front-of-balcony location 
will sell the additional seats without entailing any sacrifice 
in seating capacity. 

One objection advanced as against the main floor location 
is that it necessarily restricts the size of the projection 
room. True, but immediately below usually is plenty of 
available room in the basement, in which re-winding can be 
done, repairs made and where motor generators, etc., can be 
located, the room below being connected with the projec- 
tion room by an incline or a stair. 

The main floor location is of course usually only available 
in theatres planned to accommodate it, because the meth- 
ods for disposing of smoke and gas must be taken care of 
in the structure of the house. Otherwise it would be diffi- 
cult, if not impossible, to install the necessary ducts without 
sadly marring the beauty of the theatre. 

Summed up, the rear of the auditorium at the top of the 
balcony is usually a miserable projection room location, 
from the viewpoint of excellence in screen results. It gen- 
erally gives projection pitch far in excess of that permis- 
sible in good practice. It usually gives a too-long projec- 
tion distance, which operates to produce heavy waste of 


light in the projector optical system, as well as to make im- 
practical the maintenance of maximum sharpness in focus 
of the picture. The only advantages of such a location are 
ease of ventilation, ease of taking care of smoke in case of 
fire, ease of sound-proofing and cheapness of installation. 
In other words it barters high-class screen results for 
cheapness of installation and ease of operation. 

The front-of-balcony location costs more to install, but 
should give an almost ideal projection distance. If properly 
installed its fire dangers are purely imaginary, its thorough 
ventilation entirely practical and it may be depended upon 
to provide a projection angle well within the permissible 
limit. Special projector tables, Fig. 81 A, will automatically 
take care of displacement of picture through sag of balcony 
under load. 

The main floor location is entirely practical, from any and 
every viewpoint. Its only legitimate objection is that it is 
more costly of installation, and reduces seating capacity, 
though in the long run it may be depended upon to produce 
quite sufficient revenue to more than compensate for the 
seating space it occupies. 

The following* may be considered as the essentials of a 
first-class up-to-date projection room: 

(A) It must be so located that a point central between 
the two projector lens ports will be exactly centered with 
the center of the screen sidewise, and its height above the 
center of the screen must be such that the distortion of the 
picture will in no cases exceed 5 per cent, of its normal or 
undistorted height. 

(B) The minimum distance of the projectors from the 
screen should be such as will call for the use of a projection 
lens of not less than 4-inch E-F. Anything less than this 
focal length will make it either very difficult, or impossible 
to secure sharp definition all over the screen without reduc- 
ing the working opening of the projection lens, which 
means loss of light. On the other hand the distance from 
the lens to the screen may be as much as 250, or even 300 
feet, though we would advise against such an attempt and 
very strongly recommend that the projection distance be 
kept within a maximum of 75 or at most 100 feet, since long 
projection distance means loss of light in the optical system 
and more or less tendency to lack of sharpness in the pic- 
ture through inability of the projectionist to see it clearly. 

(C) The projection room must be absolutely fireproof, 


which includes not only the construction of the room itself, 
but the shuttering of its ports and the providing of suffi- 
cient vent pipe area to carry away all the smoke and fumes 
of burning film. 

(D) The projection room must have a very solid founda- 
tion, since the least vibration in the floor will inevitably 
produce vibration of the picture upon the screen. 

(E) The projection room must be as nearly as possible 
soundproof, to the end that the noise of the projectors, the 
rewinder, and the motor generator set or transformer, as 
well as the conversation sometimes necessary between the 
projectionist and his assistant be not audible in the audi- 

(F) The lighting of the projection room must be such 
that in case of trouble the room may be instantly and bril- 
liantly illuminated, to the end that repairs proceed with 
maximum speed. The lighting must, however, be so ar- 
ranged that the projectionist may either extinguish all 
lights, or else greatly dim the illumination by means of a 
switch located within convenient reach from his position be- 
side either one of the projectors. 

(G) The color of the walls and ceiling is important. The 
optically correct color for the interior of projection rooms 
is a non-gloss dead black, but where this is objected to a 
very dark bronze green or a dark brown or chocolate may 
be substituted with satisfactory results. The important 
thing is that the best possible view of the screen is had 
when the projection room is dark, and the darker it is the 
better will be the view of the screen. 

(H) The projection room should be reasonably easy of 
access, preferably not opening directly into the auditorium. 
It should be reached by a stairway rather than by a ladder. 
If it does open directly into the auditorium, then the stair- 
way or ladder should be surrounded by some sort of par- 
tition so that in case of fire the projectionist may leave the 
room without allowing a cloud of smoke to escape into the 
auditorium to terrify the audience. 

(I) The projection room should be large enough for rea- 
sonable comfort, allowing not less than two feet in the 
clear behind the projectors, after they have been set far 
enough back from the front wall, so that the projectionist 
may pass between the lens and the wall, with not less than 
six feet in width for a single projector and 3 feet additional 
for each additional projector, stereopticon or spot light. 


The ceiling should be as high as practicable — the higher the 
better, within reason of course. In any event 78 inches from 
floor to ceiling should be the absolute minimum. 

(J) All openings must be equipped with fireproof shut- 
ters or doors which will close quickly and automatically in 
case of fire, except the vent flue, which must be unobstruct- 
ed if there is a fan; if of the open type then it must have a 
damper weighted to remain normally open, as will be here- 
inafter explained. 

(K) There must be a vent flue or flues leading preferably 
as nearly as possible directly to the open air above the 

(L) All wires must be in conduit, and the conduit system 
must be thoroughly grounded. Fuses and switches should 
be in metal cabinets or cabinets built into the wall and cov- 
ered with a metal facing, except in cases where a regular 
switchboard is employed. Conduits should, where possible, 
be built into the walls, and conduits leading to the pro- 
jectors should be carried under the floor to a point immedi- 
ately beneath the lamp house of each projector. 

(M) The projection room must contain nothing except 
those things necessary to the work of proiection. 

(N) There should be proper tool racks, and a separate 
closet for each projectionist's clothes and tools ; also either 
in the projection room or immediately adjacent thereto 
should be a substantial work bench equipped with a sub- 
stantial metal vise and a small anvil, which two last named 
may be combined in one. 

The switches and apparatus should be so arranged that 
they will be easy of access to the projectionist, both for 
manipulation and for repair. Making things unhandy for 
the projectionist is one of the most expensive things we 
know of. 

(P) It should contain only the most up-to-date apparatus, 
which same must be kept in the best possible condition. 

(Q) The projection room must have observation ports of 
such size that the projectionist may have a clear, unob- 
structed view of the entire screen, either when seated or 
standing in working position beside his projector. 

(R) The exterior of the room should be as inconspicuous 
as possible; that is to say, it should be decorated to har- 
monize with the rest of the theatre if it projects into or oc- 
cupies a position in the main auditorium. 


(S) If good results are to be expected and those results 
are to be had with a maximum of efficiency, the projection 
room must be placed in charge of a thoroughly competent 
reliable staff of projectionists, who are possessed of both 
practical and technical knowledge of the art of projection, 
and who are able to supplement their knowledge by a good 
fund of horse sense. No applications for position as pro- 
jectionist should be considered unless the applicant can 
show that he has served at least one year's bonafide appren- 
ticeship in a projection room. 

(T) Either in the projection room or immediately adja- 
cent thereto should be a wash bowl with running water. In 
handling carbons and in oiling the projectionist gets more 
or less grime on his hands, and unless this be washed off 
some of it will adhere to the films, to their damage. It is 
also imperatively necessary that toilet facilities be provided. 

(U) There should be a telephone to the manager's office 
and, under some conditions, to the orchestra pit and stage. 
This is essential to modern practice. It should be a house 
phone only, not connecting with outside telephones, though 
an arrangement may well be made for such connection, but 
through the manager's office only. 

(V) Proper tanks must be provided for storage of films 
when not in use. These tanks should be such as will (a) 
provide a separate fireproof compartment for each reel. 
(b) Each compartment should close automatically by grav- 
ity, (c) Top of tank should be so shaped that it will not 
serve as a shelf for reels of film or other things, (d) Place 
should be provided for moisture-containing sponge. 

(W) In case the houselights are not handled from the 
projection room, there should always be a switch therein 
by means of which the projectionist will be enabled to light 
the auditorium instantly in case of serious trouble. 

The foregoing constitutes what might be termed the fun- 
damental essentials of projection room construction and 
equipment, but in addition a detailed explanation of the 
various things is necessary. 

PROJECTION ROOM DOOR.— The door of the projec- 
tion room must not be less than two feet wide by six feet 
in height. It must, of course, be built of fireproof material. 
Three-eighths inch asbestos mill board on both sides of a 
steel frame is perhaps best. The door may be either hung 
on hinges, in which case it must always open outward and 
be held normally closed by a substantial spring, or, very 



much better, it may be a sliding door so arranged that it 
will be normally held shut by gravity. 

This latter idea is illustrated in Fig. 82, in which is a door 
constructed as above set forth, hung on an inclined track. 
Such a door is easy of manipulation and not expensive in 
first cost. It will always remain closed unless it be deliber- 
ately fastened open. 

Figure 82. 

THE FLOOR. — The foundation for the projection room 
floor will, of course, vary with the circumstances of the in- 
stallation, but it must be such as will hold a floor absolutely 
without movement or vibration of any kind, because if the 
projection room floor moves or vibrates, then the projector 
itself will move and vibrate, which means that the picture 
on the screen will move or vibrate, and this latter may be a 
very serious matter, indeed. 

Suppose the projection room floor to vibrate evenly all 
over precisely 1/16 of an inch. This would mean that the 
whole picture would move up and down on the screen exact- 
ly that much, which would, if the vibration was only occa- 
sional, probably be imperceptible, but would, if the vibra- 
tion be rapid, have the effect of injuring the definition of 
the picture. But suppose the floor vibrate in such manner 


that the condenser be moved up and down l/64th of an inch 
more than the projection lens. This would set up a condi- 
tion such as is shown in Fig. 83 and the effect on the picture 
on the screen would increase in proportion as the distance 
from the lens to the screen increases. In Fig. 83 A is the 
light source and B the projection lens. If A be moved 
down l/64th of an inch, B remaining stationary you will 
readily see that the movement of the light beam at a point 
say 100 feet away will be considerable. The dotted line 
illustrates the result. 

Modern practice is to provide a solid foundation upon 
which is placed not less than six inches of a rich concrete, 
well tamped down. On this is placed a top dressing of 



Figure 83. 

cement from .5 of an inch to one inch thick, essentially of 
the same composition as is used for side walks. 

WARNING. — An enormous amount of damage <s done 
both to machinery and to film by improper mixing of the 
top dressing of projection room floors, or by the use of poor 
cements. If the top dressing be not properly mixed, or if it 
be of poor material, it will constantly wear off, and the re- 
sultant dust is a very fine abrasive powder. It gets into the 
bearings of projectors, motors and generators and wears 
them out very rapidly. It gets on the film and is one of the 
greatest rain producers known. We have seen many pro- 
jection rooms in which an improper top floor dressing was 
shortening the life of the machinery enormously, as well as 
doing immense damage to the alms used therein. 

FLOOR DRESSING.— Where such a floor exists it is pos- 
sible to apply a preparation which will stop the trouble, or 
the floor may be painted with a good floor paint. Exhibit- 
ors will do well to remember that a projection room cement 
floor which slowly disintegrates into dust is about as great 
a damage producer as they can have in their theatre, and 
one great trouble whh this particular thing is that the dam- 
age done is so nearly imperceptible that neither the ex- 
hibitor or the projectionist realizes how serious it really is. 

One of the be'jt possible coverings for a projection room 
floor is a heavy matting made of a cork composition, such 


as battleship linoleum, but the plain cement finish will not 
produce trouble if it is made from a proper mixture of good 
cement. A floor built of concrete as described, finished with 
a proper top dressing of cement, will to all intents and pur- 
poses be just one solid block of stone, and you will have no 
vibration at all, because a thing of that kind is altogether 
too heavy to vibrate. 

WALL CONSTRUCTION.— Where there is no objection 
to weight, cement or brick is very good for wall construc- 
tion. Either is, of course, thoroughly fireproof. Hollow tile 
has distinct advantages, however. If properly and carefully 
laid in rich mortar, weir tempered with cement, it is just as 
good for the purpose, viewed merely as a wall, as cement 
or brick, and is much more nearly soundproof than either ; 
also it does not act as a heat reservoir as does brick and 
cement. Where hollow tile is used it should be well plastered 
on both sides, either with one of the hard-setting patent 
plasters, or with a strong lime mortar strongly tempered 
with cement. 

When either of the three before named forms of con- 
struction are used the ceiling may be of the same material, 
carried in the usual way, between I beams. 

It is not the purpose of this work to enter into details of 
construction. That is the business of the architect. Our 
intent is only to point out those things which practice has 
proven to be satisfactory. 

BUILDING IN CONDUIT.— All projection room circuits 
must be in conduit, and where cement construction is used 
it is much better that the conduit be placed in position as 
the walls are built, so that it will be imbedded in the walls 
themselves. Where brick construction is used it is possible 
to leave grooves in the wall, so that the conduit can be 
placed therein and afterward imbedded in plaster. The 
bricklayer mason may charge extra on account of the 
trouble involved in doing this, but it nevertheless is prac- 
tical and worth while, because the conduit is then not only 
held firmly in place, but becomes a part of the wall itself. 

PORTS,— Each projector must be provided with a lens 
port and one observation port. Each stereopticon must 
have one observation and one lens port, but a spot light 
need only be provided with one large port. Of course if the 
projector be provided with a stereopticon attachment, then 
a separate port must be provided for slide projection. 



LOCATING PORTS.— The way architects locate lens and 
observation ports usually results in handicapping projection 
forever after. The average architects, if we are to judge by 
what they do, have little, if any, comprehension of the im- 
portance of properly located ports of proper size. They 
locate a lens port somewhere near right, and an observation 
port which, four times out of five, is nothing short of an 
outrage, and which makes high class screen results highly 
improbable, if not entirely impossible. We strongly recom- 
mend the following procedure in planning the projection 
room front wall. 

Lay out the openings as per Fig. 84, in which A and B are 



















Figure 84. 

the projector lens ports, C and D the projector observation 
ports, E and F respectively the stereopticon lens and obser- 
vation ports, the latter of which may be made any size 
desired not less than ten inches square. 

It will be noted that the projector lens ports are indicated 
as twelve inches square. This is for the reasons given fur- 
ther along. If it is not desired to follow the procedure we 
shall indicate for filling in the lens ports, then their size may 
be reduced accordingly, but the filling-in plan is most satis- 
factory in the end. 

The distance between ports A and B is the absolute mini- 
mum consistent with good work and decent conditions. It 
should never be less, and should never be more where the 
distance of projection and picture size be such that a pro- 
jection lens of less than 6-inch E-F is required. When the 
projection lens E-F exceeds 6 inches, the distance between 
ports A and B may be increased gradually as the E-F of 


required lenses increases, until with an 8-inch E-F lens it 
is possible to have as much as five feet between projector 
lens centers, though four feet gives ample room and should 
not be exceeded. 

The distance between stereopticon and spot may be re- 
duced, if necessary, as may also the distance between the 
stereopticon and left hand projector, though we do not 
advise this latter if it can in any way be avoided, as it sets 
up a bad condition. 

Ports A and B must, of course, be spaced equi-distant 
from the center line of the screen. 

The distance from port B to the wall also is intended as 
an absolute minimum. It is, in fact, too little. When it is 
possible, at least, another foot should be allowed. 

CAUTION. — The distance of the center of projector ob- 
servation ports C and D above the floor is shown as 60 
inches, which is about right, everything considered, for 
level projection and the average man, but if there is consid- 
erable pitch in the projection the ports will be too high. It 
will be found very satisfactory to make the projector ob- 
servation ports 16 inches wide, with their centers 60 inches 
from the floor, and then lower them four and one-quarter 
(4.25) inches for every five degrees pitch in the projection. 

Thus: If there be a 20 degree pitch in projection, then 
the centers of the sixteen-inch-square observation ports 
should be (4.25x4=17) 60—17=43 inches from the floor. If 
there is considerable pitch in projection it will also be 
necessary to lower the lens ports. 

It will be observed that ports A and B are 16 inches 
square, and that port E is 18 inches high by 8 inches wide, 
which is, of course, far in excess of the actual requirement. 
The center of the lens of the average projector is about 48 
inches from the floor when the projector is level. The idea 
in making the lens port opening of the wall 12 inches square 
is that after the projectors are placed in position and the 
light properly centered on the screen the opening may be 
filled in as per Fig. 85, in which A is an asbestos mill board 
Y% inch thick set into the opening and flush with the outside 
edge of the wall. This is only placed in the port after pro- 
jector has been set in place and the light has been centered 
on the screen. After placing board A in position, turn on 
the light, which will form a spot on board A. With a 
pencil, mark around the outer edge of the spot and then 
cut a hole in the board just a bit larger. Next a similar 





, // ///// 

K /f,it" 

' , / / / 

board is placed as per C and the hole marked in the same 
way and cut out, after which the two boards are placed in 
position and bolted together, a wood spacer being used as 
shown. We thus have a lens port exactly the size of the 
requirements of the local condition. F-F are bolts holding 
the whole thing together. This port filling is effective and 
its cost is small. Stereopticon lens port E should be filled 

in in the same way. 
Flanges of asbestos 
mill board may be 
added, as per dotted 
lines, if desired. 

C and D, Fig. 84, 
are, you will ob- 
serve, 16 x 16 inches 
in dimension, with 
their horizontal cen- 
t e r s located 60 
inches from the 
floor line, and the 
vertical center 18 
inches from the cen- 
ters of the lens 
ports. Observation 
ports should, under 
no circumstances, be 
less than 12 inches 
wide, and 16 is much 
better. Anything less 
than 12 inches com- 
pels the projection- 
ist to put his eyes 
right up against the 
port in order to get a 
clear view of the 
screen. The average 
layman, or even the 
average projection- 
ist, does not realize how true this is, or what an effect the 
narrow observation port has in injury to the definition of the 
picture on the screen, by reason of the fact that the projec- 
tionist just simply cannot see clearly through a small hole. 
Where a local law limits the size of observation ports it is 
entirely practical to cover a large port with a sliding shutter 
in which an opening 12 inches wide by 6 or 8 inches high has 

Figure 85. 



been cut. This sliding shutter must be so arranged that 
the bottom of the opening in it may come down to the 
bottom, and the top of the opening raise to the top of the 
hole in the wall. It should be arranged with a counter- 
weight so that the projectionist may move the opening up 
or down to the most convenient position. The plan is illus- 
trated in Fig. 86. 






/KM/*T*&i.e 4*irry>0-* 

Figure 86. 

The important point in making an observation port of 
generous size is that unless the projectionist watches his 
screen constantly there will be faults of various kinds in 
the projection, and most certainly there will be little if 
any of that regulation of the speed of projection which is 
so necessary to high class work. Moreover, unless the port 
be of ample size, the definition of the picture on the screen 
will not be as sharp as it should be, which means added 
and unnecessary eye strain to the patrons of the theatre. 

GLASS OVER PORTS.— It is entirely practical to place 
glass over the observation ports, provided the right kind of 
glass be used, and further provided the glass be properly 
installed and KEPT CLEAN. An old photographic plate 
with the emulsion cleaned off is the best glass to use, though 



any high grade glass will do. Sec Page 290 for directions 
for removing photographic emulsion. 

Where glass is placed in an observation port it should be 
set at an angle from the vertical, which will serve to kill 
the reflection from its polished surface. It must either be 
made easily removable, or be hung on hinges so that it may 
be readily cleaned on its outer surface. If the glass be set 
at an angle, and the port be surrounded by a shadow box 
painted black on the inside, as per Fig. 87, the reflection 

from the glass will be en- 
tirely killed and the view 
of the screen made very 
much better. 

In one of the San Diego, 
California theatres the 
author saw what seemed 
to be the ideal observation 
port. It was located be- 
tween the projectors, was 
about 30 inches square and 
was covered with plate 
glass, in the center of 
which a circle 10 inches in 
diameter had been cut out. 
With such a port it seems 
possible to have the pro- 
jection room at all times 
fairly well lighted, pro- 
vided the lights be prop- 
erly placed, and at the 
same time to have a most 
excellent view of the 

GLASS IN LENS PORTS.— Many projectionists now use 
glass in both the lens and observation ports. The glass 
over the lens port need not necessarily do any harm, insofar 
as the definition of the picture be concerned, but it does 
cause considerable loss through reflection, particularly if 
it is not kept scrupulously clean. If the lens port be re- 
duced to the actual requirement of the beam there is seldom 
any necessity for covering it with glass, since the opening 
will be very small. Our advice is to reduce the lens ports 
to the actual requirement, as per Page 309, and omit the 

Figure 87. 


SMALL PORTS AND THE LAW.— In some states the 
size of projection room ports is limited by law, and the 
limitation is such that high class projection is prevented, 
since it is impossible for the projectionist to secure a really 
good view of his screen, even when right up against the 


In case of fire, smoke will make its way through a small 
opening practically just as quickly as it will through a large 
one, and if the audience once gets sight of smoke the 
damage is done and the panic started, if there is going to 
be one. The escape of smoke from the projection room is, 
however, supposed to be prevented by the fire shutters and 
there will not be the difference of l/25th of a second in 
the time required for a shutter to drop over an opening 16 
inches square and the time required for a shutter to drop 
over an opening 4 inches wide by 12 inches high. 

Then, too, the larger shutter being much heavier is less 
likely to stick than the small one, which actually places the 
balance on the side of the large opening, insofar as con- 
cerns safety. 

If law makers and inspectors would pay more attention 
to the proper construction of port shutters and the proper 
placing of the fuses in the line's which support them, leaving 
the size of the opening to take care of itself, the result, in- 
sofar as concerns the safety of the public, would be greatly 
improved, and the enormous handicap against projection 
caused by small observation ports would be removed, to the 
great benefit of results on the screen, the lessening of eye 
strain for the spectators and the increasing of the enjoy- 
ment of the show by the public. 

SPOT LIGHT PORT.— The spot light port, if one there 
be, should be located with its center about 4 feet 6 inches 
above the floor line. The opening should be 16 to 18 inches 
in diameter, square or round, as preferred. 

PORT FIRE SHUTTERS.— Every observation port, lens 
port and spot light port should be provided with a fire 
shutter. The best material from which to make these 
shutters is asbestos mill board, Y% of an inch thick. Some 
authorities are satisfied with 16-gauge sheet metal, which 
will serve very well, though it is not, we believe, all things 


considered, as satisfactory for the purpose as asbestos 

The proper installation of port shutters, together with an 
adequate vent flue and thoroughly fireproof walls offer not 
only adequate protection from fire damage to anything out- 
side the projection room, but also against the probability of 
alarm on the part of the spectators should a fire occur in 
the projection room. This latter desirable end will, how- 
ever, not be accomplished unless the port shutters be so 
hung that they will close the instant fire starts. This is of 
absolutely supreme importance. It is very, very seldom, if 
ever, that a projection room fire does any injury to any- 
thing outside the projection room itself. The damage is 
practically always caused by the panic which almost in- 
variably follows an alarm of fire where an audience is 
gathered. Now that all projection rooms are fireproof, it 
may be stated as a fact that, 

Barring panic, there is absolutely no danger of any kind 
whatsoever to an audience or to any individual thereof 
through a projection room fire. 

A splendid result would be accomplished if all theatres 
were required to run a slide reading something like this> 
before each performance, for a period of six months: 

"The projection room of this theatre is thoroughly fire- 
proof. In the improbable event of a film fire there is 
absolutely no danger to the audience, as in no possible event 
could anything more than possibly some smoke reach the 

The foregoing might possibly be improved as to its word- 
ing, and is intended only as a suggestion. 

It is a deplorable fact, however, that a large proportion 
of the average audience becomes raving maniacs the in- 
stant there is an alarm of fire. Given a glimpse of fire or 
smoke you may depend upon them to go stark raving mad, 
pile up in a heap and kill each other either through tramp- 
ling or suffocation. 

We desire to strongly impress upon architects and ex- 
hibitors and public officials the fact that it is entirely prac- 
tical and feasible to prevent the audience from having any 
glimpse of either fire or smoke when a film fire occurs. In 
order to accomplish this, however, port shutters must be so 
fused that they will automatically close every opening in 
the projection room within three or four seconds of the time 
a fire starts. 



The port fire shutters should be so arranged that they may 
all be dropped by the projectionist with one motion, but 
depending upon him is by no means a safe thing, since the 
projectionist is but human. When the film catches fire he 
is likely to become more or less excited, and one never can 
tell what an excited man will do, or what he won't do. We 
therefore emphasize the fact that. 

Port shutters should be so fused that they will auto- 
matically close all the ports within three or four seconds of 
the starting of a fire at either projector, at the rewind table 
or the film storage can. 

This latter proposition hinges entirely upon the kind and 
location of the fuse links. All shutters should be held by 


one master cord, and that master cord should contain fuse 
links, preferably of film. But whether the fuse links be of 
film, or the regulation metal fuses should NOT be located 
4 or 5 feet from the probable sources of fire, but as 
closely as possible to (a) each projector upper magazine, 
(b) the film storage magazine and (c) the rewind table. If 
metal fuse links are used they should be of the 160 degree 
fuse metal, or the most sensitive fuse metal obtainable. 
One method of using film links is shown in Fig. 88. 

There are several patent port shutter supporting and 
releasing devices made, and they are all of them good, pro- 
vided the fuse link be located where it will do real service. 

There is one plan, however, the invention of F. E. Cawley, 
Mason City, Iowa, which is of such general excellence that 



it deserves description. Its chief point is illustrated in 
Fig. 89, in which A is a round shaft hung in suitable bearings 
attached to front wall at suitable height. To it is attached 
lever B, as shown. In shaft A holes about % inch in 
diameter are drilled, passing clear through. These holes 
are indicated at G-G-G. There must be one over the center 
of each port opening. To each port shutter a suitable cord 
or wire is attached, at the other end of which is an ordinary 
cotter pin about an inch long. The same purpose would be 



Figure 89. 

served by a pin, as at D, and a small harness ring. The 
action is as follows : 

Lever B is raised to horizontal position, which must also 
bring holes G-G-G to horizontal position, and a master cord 
is attached to the hook at its end. This cord may be carried 
down to proper position at each projector, also down over 
the film storage tank and the rewinder table, being fused 
with link or metal fuses, or both, at proper places. It ter- 
minates in a metal ring designed to be hooked over a head- 
less spike just inside the projection room door. 

It will readily be seen that when the master cord is re- 
leased, either by a fuse or by removing the ring from the 


spike, lever B will fall, and in so doing will rotate rod A 
one-quarter of a turn, so that holes G-G-G will be vertical, 
whereupon pins C will be released (they must fit very 
loosely in the hole) and the shutters will drop of their own 

The superiority of this plan is that it combines the great 
advantage of fusing at as many places as may be desired 
(the master cord really need never be disturbed, except in 
case of emergency, as a fire) and the ability to raise or 
lower each shutter entirely independently of every other 
shutter. Also it can easily- be made by any competent black- 
smith or machinist. The scheme has our hearty commenda- 

AUTHORITIES IN ERROR.— As a general proposition 
authorities permit or even demand the locating of port 
shutter fuses near the ceiling. Their theory is that since 
heat rises the air at or near the ceiling will become heated 
very quickly. This is true, but where a panic may hinge on 
a matter of two or three seconds the plan is a very bad 
one. It is all right to locate fuses near the ceiling, but 
there must be other fuses near the probable source of fire. 
Fuses are cheap, and it won't cost much to use a dozen of 
them. On the other hand a fire panic may be very ex- 
pensive in human life. 

Let it be clearly understood that in the foregoing it is 
also presumed that a proper arrangement will be made 
whereby the projectionist may himself drop the port shut- 
ters, which should be absolutely the first thing he does, 
after dropping the douser or pulling the switch, when a fire 
starts, since the safety or the audience is paramount to 
everything else. 

shutters drop, their bottom edge should rest either on rub- 
ber or felt. The most important purpose of the port shutter 
is to prevent the audience from even knowing there is a fire 
in the projection room. If from four to six heavy shutters 
drop with a crash, the effect is to instantly draw the at- 
tention of the entire audience directly to the projection 
room. The pad is designed to avoid this very thing. Of 
course in theory the shutters should be dropped gently, but 
when a fire occurs and the shutters drop automatically 
through the burning of a fuse, gravity won't act very gently, 
and even if the projectionist drops them he has not, under 
the circumstances, time to be very gentle. 



Fig. 90 indicates a proper method of padding the shut- 
ters to prevent a noise when they drop. 

VENTILATION of the projection room serves three pur- 
poses, viz. : to provide fresh air to the men working therein, 
to keep down the temperature and to provide means for 
carrying off all the smoke and gases generated in case of 
film fire. 

The vent flue should, wherever possible, pass directly 
from the projection room ceiling through the roof to the 
open air, its top being not less than 3 feet above the roof 
and protected by a suitable hood to keep the rain from 
beating in. The open vent flue is neither safe nor de- 
sirable, because of the fact that under some conditions it is 

Figure 90. 

quite possible, and even probable, that the draft through an 
open vent would be downward instead of upward. This is 
especially true in some locations when the wind is in certain 
directions, as any housewife who has had experience with a 
smoking chimney can testify. 

The laws of some cities or states stipulate a certain size 
vent flue for a certain size projection room, and another size 
flue for a different size projection room. This might be 
correct if only the purposes of ventilation were to be 
served, but one of the important offices of the vent flue is 
to prevent panic by carrying off all the smoke and gases, 
and film burning in a small room makes just as much smoke 
and gas as it does in a large one, hence a small room should 
have just as big a flue as a large one. 


If the vent flue or pipe be of the open type it should have 
an area of not less than 288 square inches, regardless of 
the size of the room. 

Where a vent flue depending upon a fan for its action is 
used, the fan should not be less than 24 inches in diameter. 
Where fans are used it is an exceedingly good practice to 
install two vent pipes and two fans instead of one, so that 
in case one of the fans gets out of order there will still be 
the second one to fall back on. This may seem like rather 
an expensive precaution, but since the pipe containing the 
additional fan may join the pipe of the other fan, the added 
expense will be largely that of the second fan, and where 
the safety of the audience is concerned expense should not 
be considered. 

It is essential that the vent flue, if made of metal, be 
thoroughly and completely insulated from any inflammable 
substance throughout its entire length, since it is likely to 
get very hot if there is a serious fire in the projection room 

With proper means provided for the egress of smoke 
and gas, when a film fire occurs, the projection room with 
fireproof walls will be nothing more nor less than a hug-? 
stove, the draft being inward around the cracks of the port 
shutters and door, and outward through the vent flue; s* 
that no smoke or gas will in any event show in the audi- 
torium. Hence the audience will never know there is a fir* 
in progress, even though their attention be attracted direct% 
to the room by the stoppage of the show. 

In addition to the vent flue, there must, for the sake of 
establishing healthful conditions through proper ventilation, 
be means provided for the ingress of fresh air. The pro- 
jection room is often (we might almost say usually) located 
immediately under the roof of the building, and in con- 
sequence is, in summer time, very hot with "natural" heat. 
Add to this the heat generated by the powerful arc lamps, 
as well as perhaps one or two rheostats, and you have a 
condition which makes good ventilation absolutely impera- 

In connection with this it must also be remembered that 
air taken from the auditorium will not be pure air, and 
where the projection room is near the ceiling will be the 
warmest air in the theatre. 

It must also be remembered that if it is taken in through 
the lens and observation ports a draft is created through 
them which blows directly upon the projectionist, a fact 


which has sent many men to their death through pneu- 
monia brought on by "cooling off" in the draft before an 
observation port after some rapid repair work in an over- 
heated room. 

Where the theatre is provided with a ventilation system 
thorough ventilation for the projection room may be had 
by including it in the ventilation scheme. Where there is 
no such ventilation, then there should be inlet air ducts 
from the outer air, these inlet openings to reach the pro- 
jection room near the floor line. The Massachusetts law is 
very good in this respect. It reads as follows : 

Projection rooms to be provided with an inlet in each of 
the four sides, said inlets to be 15 inches long and 3 inches 
high, the lower side of the same not to be more than 2^ 
inches above floor level. Said inlets to be covered on the 
inside by a wire net of not greater than ^-inch mesh; 
netting to be firmly secured to the asbestos board by means 
of iron strips and screws. In addition to the above there 
shall be an inlet, in the middle of the bottom of the pro- 
jection room, if possible ; otherwise in the side or rear of 
the projection room, not over 2y 2 inches from the floor. 
Said opening to be not less than 160 square inches area 
for a No. 1 projection room, 200 square inches area for a 
No. 2 projection room, and 280 square inches area for a 
No. 3 projection room ; connected with the outside air 
through a galvanized iron pipe with a pitch from the 
projection room downward to the outside wall of the 
building. The opening to be covered with a hood, so ar- 
ranged as to keep out the storm, and the entrance to the 
projection room to be covered with a heavy grating over 
^-inch wire mesh, if in wall ; and arranged with damper 
hinged at the bottom, and rod or chain to hold said damper 
in any position. Mesh and gratings to be securely fastened 
in place, those in the walls to be bolted on as specified for 
the smaller inlets. 

Note — No. 1, No. 2 and No. 3 refer to the size of rooms. 

the box office receipts of a motion picture theatre \o a very 
srreat extent depend upon the excellence of results upon 
the screen, the wise exhibitor will bend every effort toward 
the attainment of high class, artistic projection, and will put 
forth everv reasonable endeavor to secure a high class, 
brilliant, flickerless picture, projected at proper speed. It 
may be put forward as a statement of incontrovertible fact 


that there is small probability of continuous high class 
screen results coming from an ill-placed, small, poorly ven- 
tilated projection room, poorly equipped with facilities and 
tools, and having inferior or badly worn equipment under 
the charge of a projectionist of mediocre ability. No one 
will, we think, dispute the proposition that the above com- 
bination would react seriously upon box office receipts. 

We believe everyone will agree that the best results will 
be had from a properly located, commodious, well ventilated 
projection room, equipped with up-to-date projection 
machinery which is kept in good state of repair, and with 
ample tools and facilities ; the whole being in charge of a 
thoroughly competent projectionist; the term "competency" 
including industry and careful attention to detail, as well as 

In this connection it is well to bear in mind the fact that 
the mere possession of knowledge counts for very little 
if its possessor is too lazy or too shiftless to apply it in 

CLOSETS. — In planning the projection room the architect 
should include sufficient cabinets, or closets, with substantial 
locks thereon, so that each individual projectionist may have 
a place to keep his private belongings, including tools. 
The projectionist should have a full equipment of tools, but 
it is rather discouraging to provide them and then be com- 
pelled to leave them at the mercy of anyone, from the 
janitor to a chance visitor, to say nothing of the other pro- 
jectionist who perhaps has none of his own, and moreover 
may not be inclined to take the best care of those belonging 
to others. 

There should also be drawers, or a closet in which to keep 
supplies, such as carbons, extra lenses, etcetera, though, of 
course, a shelf will do, and if the walls be built of cement 
it is a comparatively simple matter to provide cement 
shelves when the room is constructed. There should also be 
plenty of hooks on which to hang spare wire, etcetera. It 
is an exceedingly unprofitable thing to spend time hunting 
for a piece of wire, a tool or some needed repair part when 
something goes wrong. 

It is of the utmost importance to orderly procedure and 
rapid work in a projection room that things be kept in 
their place, but in order to keep things in their place it is 
necessary to have a place to keep them in, and that is some- 


thing it is up to the designer or architect to provide for 
when the room is built. 

If no conveniences are provided the manager cannot very 
well blame the projectionist if things are not kept in order, 
but if conveniences are provided and the projectionist does 
not keep things in order, then he is not a fit man to have in 
charge of the projection room. 

RUNNING WATER, TOILET.— It is highly important 
that there be a wash basin with running water, and a toilet 
either in or convenient to the projection room. Both of 
these are very essential, and the latter exceedingly so where 
there is only one projectionist. The basin is necessary be- 
cause often something will go wrong with the machinery 
and the projectionist will get his hands covered with oil 
and dirt in making necessary repairs. He will also get 
carbon dust on his hands when trimming the lamp, and if 
there be no means of removing this grime, the next time 
he handles the film it is more than likely that considerable 
damage will be done to it. Moreover, he is apt to soil 
everything he touches. The installation of a wash basin 
and running water is, therefore, highly important. Toilet 
facilities should be required by law, since in many cases 
the projectionist cannot leave the projection room for 
hours at a stretch. 

PROJECTION ROOM CHAIR.— Some theatre manage- 
ments will not allow their projectionist to use a chair. This 
is, we are firmly convinced, not only a mistake, but a very 
serious one. One thing imperatively necessary to high class 
projection is that the projectionist remain constantly at his 
post beside the projector. Exhibitors constantly and rightly 
complain that projectionists will not remain at the observa- 
tion port, where they belong, and that the projection in 
their theatres suffer by reason of this fact. 

We believe there is several times the likelihood that a 
projectionist who is standing on his feet will move around 
the room than there is if he is seated. The man who is 
seated at his projector in front of the observation port is 
likely to stay seated unless something happens which re- 
quires that he get up. The idea that a projectionist seated 
beside his projector cannot do just as good work as he 
would if standing up is pure nonsense. When the author 
was a projectionist he always did his work seated beside 
the projector. Standing several hours on his feet in one 
position or place would have been to him almost an im- 


possibility. He was entirely able, while seated at the pro- 
jector, to make any necessary adjustment, and to make it 
just as quickly as though he were standing. If anything 
happened it required about l/10th of a second for him to 
rise. There is neither rhyme, reason nor common sense in 
the refusal of a chair to the projectionist. The wise man- 
ager instead of refusing the chair would see that one was 
placed at each projector, and demand that the projectionist 
use it, on the theory that when using the chair he would 
"stay put" where he belongs beside the projector while the 
show is running. 

PROJECTION ROOM REELS.— For many years the Pro- 
jection Department of the Moving Picture World has recom- 
mended that theatre projection rooms be equipped with a 
full complement of projection room reels, and that the reels 
upon which the films are received from the exchange be only 
used in the upper magazine of the projectors the first time 
the films are projected (not even then if the films be first 
rewound or inspected), and on the rewinder when the film is 
rewound for the last time before shipment to the exchange. 

Excellent reason for this recommendation is found in the 
utterly wretched condition of the great majority of reels used 
by exchanges for shipping films to theatres, and in the fur- 
ther fact that many exchanges have in the past purchased, 
and at this time (1927) still are purchasing, reels too cheap, 
flimsy and rough to be fit for use on a projector, or for 
anything else, for that matter. 

A literally enormous amount of damage to film is caused 
by crooked, rough reels, and reels having a too-small hub 
diameter. Such reels are an outrage upon common sense, 
and the fact that exchanges purchase and use them, or have 
purchased and used them, is by no means complimentary to 
the good business judgment of exchange owners and man- 
agers. An exchange may "save" ten cents by winding a new 
roll of film on a bent, rough, cheap reel, and may, in the 
process, lose many times that sum by the actual, though 
not always visible damage done! to the film itself. 

At the time of the compilation of this book two really 
excellent projection room reels were presented for examina- 
tion and test; also at least one more was in process of 
making. Both those presented were of such construction that 
they deserve commendation, and both are therefore recom- 
mended to the favorable consideration of users of this book. 



FILMFAST REEL.— The reel known under the trade name 
of "Filmfast Reel" is designed wholly for use in the projec- 
tion room. It is a substantial, well constructed reel from 
every viewpoint. The patentee of the reel is Walter I. 
Tuttle, president of the Frank Mossberg Company, the 
address of which firm will be found in their advertisement 
elsewhere. The dimensions of the reel are 14^ inches out- 
side diameter, with a barrel (hub) diameter of 4^4 inches. 
In and on the hub, or "barrel" is an excellent film gripping 
arrangement, which facilitates threading by enabling the pro- 
jectionist to readily at 
tach the end of the film 
to the reel with one 

The sides of the reel 
are of heavy sheet steel, 
of a quality selected en- 
tirely with reference to 
the duty it has to per- 
form. The sides are 
heavily embossed, 
which together with the 
heavy weight metal will 
prevent kinking, bend- 
ing or warping even 
under pretty rough 
usage. The hub con- 
struction is very sub- 
stantial. It has a cen- 
tral half-inch-diameter 
stud, drilled and reamed to fit accurately on the take-up 
spindle. This stud is a driving fit into a heavy washer at each 
end, which prevents the key ways from spreading. The two 
washers are joined to each other through the side plates by 
means of three studs which are riveted into the washers. 

The barrel (hub) is attached to each of the reel sides by 
means of twelve metal ears, which same are clinched down 
tight. On the outer surface of the barrel (hub), and con- 
centric with it, is a heavy spring attached to its middle por- 
tion. To either end of this spring is attached triggers, which 
same extend within the barrel and form a gripping arrange- 
ment for the finger, as is shown in Fig. 90a. The projection- 
ist pulls on the trigger raising the end of the spring, and with 
the thumb of the same hand slips the film end under the 

Figure 90a. 



spring. It is all done with one motion and with one hand, 
and is a positive gripping of the film end. 

The "Filmfast" reel will be shipped you packed in indi- 
vidual corrugated paper cartons, which insures their receipt 
in good condition. We recommend the reel to your favorable 

tional Projector Corp., manufacturers of the Simplex pro- 
jector, is marketing a projection room reel under the above 
name. This reel appeals to us as a very practical, high grade 

proposition for projec- 
tion room use. 

In the first place, as 
will be seen in Fig. 90c 5 
the sides are of steel 
wire, about 5/64th of an 
inch in diameter. The 
loops are connected to 
the hub, which is five 
inches in diameter, in 
the way shown in Fig. 
90c and Fig. 90d, in 
viewing which you will 
understand that the 
outer shell of the hub 
(F in Fig. 90C), has 
been removed in order 
Figure 90b. to show the method of 

Filmfast reel with sides removed connecting the loops to 

showing hub construction. the hub studs, the cen- 

ter hub itself and the automatic signal device. 

The construction as a whole embodies the following sub- 
stantial points of advantage : 

(A) The reel is substantial and rigid. 

(B) It is difficult to bend, and the bending of the reel at 
one point does not affect other portions of the reel; also the 
zone which is bent may be straightened and the reel again 
be made perfect, something which, so far as we know, 
cannot be done with any sheet metal side reel. 

(C) The form of construction and the form of material 
used effectually prevents the cutting or scratching of film, or 
the cutting or scratching of the projectionists' hands, both of 
which are very common with the cheap sheet metal side reels 
now in general use. 



(D) The open construction of the reel is such that, while 
it affords ample protection to the film, it makes the reel very 
accessible for threading, etc. 

(E) The hub of the reel is the best we have yet seen, in 

Figure 90c. 

that it consists of a solid cylinder of steel, through which a 
hole is drilled to receive the spindle of the take-up or re- 
winder. In either end of this steel hub a slot is cut clear 
across the hub, thus forming a double-sided key way. This 
renders it exceedingly improbable, if not impossible that the 


key way will ever wear out, and that is one of the weak 
points in all old style reels; in fact it might be truthfully said 
that the weakest point of the many weak points in old style 
reels was the key way. 

(F) In its center, within the hub, the reel contains an au- 
tomatic dissolving signal, as will be explained further along. 

IN THREADING the reel the projectionist passes the end 
of the film under either one of studs A A, Fig. 90c, both 
of which are bright nickeled, being certain the end of the film 
is underneath, as per Fig. 90c. 






M \\ 

'"$ m ■ 


[ . ' ^^ 



^^ \$n 



; |I 

'4 \ 

\ A l 

■Pjr ^ 

d ■ A 





1 \ 

^l j 

; Figure 90d. 

IMPORTANT. — The time you will have for making the dis- 
solve after the signal is given, depends, within reasonable 
limits, upon the length of the end of the film pulled under 
stud A. The longer this end the more time there will be 
after the signal sounds before the screen goes white. 

When you have end E as long as you wish, you must give 
the reel at least one full turn in the direction it normally 
runs, which not only locks the film by friction upon itself 


but docs another important thing, viz.: Mounted on stud 
B is pivoted arm C, to which the gong hammer is attached, 
as shown. When the reel is given a turn or more to lock 
the film to the hub by friction, it pulls arm C into position 
which holds the gong hammer in the position shown in Fig. 
90d. The action is as follows : Mounted within the shell of 
the hub is a 4-inch diameter gong, marked D in Fig. 90c. 
So long as arc C is held in position, as per Fig. 90d, by the film, 
the hammer cannot stri e the gong, but when the last layer 
of film unwinds in process of projection, arm C is released 
and thereafter at every revolution of the reel (and the reel is 
then revolving rapidly) the hammer will strike the gong, 
which is the signal for immediate change-over — in other words 
a signal to immediately dissolve and change to the other pro- 

We can recommend the Simplex Automatic Signal Reel to 
the favorable consideration of our readers. 

PROJECTION ROOM SUPPLIES.— It is impossible to 
imagine a more foolish and utterly mistaken policy on the 
part of a theatre management than niggardliness in the 
matter of projection room supplies. The suspicion held by 
some theatre managers that if they do not constantly "sit 
on the lid" in the matter of projection room supplies waste 
will occur, is as foolish as it is unworthy. 

The projectionist who cannot be trusted to be careful 
and economical with supplies is not a fit man to be in the 
projection room at all. 

Many theatre managers, however, have a mistaken idea 
of what constitutes economy in supplies. 

The competent projectionist does not wait until a part 
breaks down entirely, thus perhaps stopping the show until 
repairs are made, nor does he wait until a part is so badly 
worn that the screen result is made to suffer. He renews 
parts before a break comes or before there is damage to 
the screen result. 

From any and every viewpoint it is false economy to at- 
tempt to get the last possible bit of wear out of projection 
room equipment. Take, for example, asbestos wire lamp 
leads. Entirely too many projectionists use their lamp 
leads, particularly that portion inside the lamp house, alto- 
gether too long. Inside the lamp house the wires are sub- 
jected to increasing heat from the arc as they approach 
nearer to it, and as the temperature of metal rises its re- 
sistance also rises. Copper oxidizes under the action of 


heat, and where a wire which is working close to capacity 
electrically is subjected to the action of heat from an out- 
side source, the effect is to raise the resistance of the wire, 
thus lowering its carrying capacity and setting up still more 
heat and rapid oxidization and deterioration. In a very 
short time the strands of the asbestos wire inside the lamp 
house turn brown, then dark brown. 

If you strip back the insulation you will probably find the 
wire to be discolored for a considerable distance. Under 
this condition its resistance is very high, and since resist- 
ance means loss which is registered on the meter, the wire 
is consuming within itself, by reason of its high resistance, 
wattage which in a few hours' time will more than equal 
the cost of the wire, modified, of course, by the fact that if 
current is taken through an adjustable rheostat there may 
be no actual loss, as that much less resistance will be re- 
quired in the rheostat. But the condition is a bad one 

Yet it is a fact that many theatre managers, lacking 
knowledge of such matters themselves, and unwilling 
to trust the knowledge of their projectionist, protest against 
the cutting off of burned wire, and demand that it be used 
longer. They are "saving" a few cents in wiring deplace- 
ment at the expense of a great many cents in electrical 

There is always a tendency to use the intermittent 
sprocket of the projection machine too long, with conse- 
quent damage to the film and to the screen result, and any- 
thing which damages the screen result is expensive, because 
the public patronizes a theatre in proportion to the pleasure 
it gets out of what it sees on the screen. 

I mention these two examples merely as being typical, and 
place them in evidence as showing that it does not pay to be 
too economical in the matter of projection room supplies ; 
also as proof that lack of knowledge often causes a theatre 
manager to practice that which is in fact false economy; in 
other words to practice economy which is as a matter of 
fact not economy at all, but exactly the opposite. 

It is the part of wisdom for the theatre manager to em- 
ploy only projectionists in whose ability he has at least a 
reasonable amount of confidence, and having done so to 
allow them a reasonably free hand in the matter of pro- 
jection room supplies. 

As a matter of business the theatre manager may very 


well require his projectionist to keep a record of (a) all 
supplies purchased, (b) make weekly report of repairs and 
replacement, and (c) that all old machinery parts replaced 
be delivered to the manager's office. 

PROOF. — In proof that it does not pay to be too economi- 
cal in supplies, let us cite the following: We believe almost 
any projectionist will agree to keep both projectors in 
first class condition for a year for the sum of $150, this not 
to include ordinary deterioration of the machine as a whole, 
but merely a sufficient replacement of worn parts to keep 
the projectors in first class repair. $150 a year amounts to 
41 V- 10 cents a day. A theatre of 700 seating capacity giving 
four shows a day has 2,800 seats to sell, or 2,800 admissions 
to dispose of each day. Supposing the admission price to 
be 15c, it would only require the price of less than three 
seats to keep the projection machines in absolutely first 
class mechanical condition, and 

Is there a theatre manager on earth who does not realize 
and know that projectors in first class mechanical condition 
will produce a sufficiently better screen result than pro- 
jectors in poor mechanical condition, to increase seat sales 
by an average of three a day out of 2,800? 

argument applies equally well to the projector as a whole. 
Let us assume John Jones to own a theatre having 700 
seats, giving four shows a day. He therefore has 2,800 
admissions for sale each day. Let us assume his admission 
price to be 20 cents. John Jones has two old type pro- 
jectors which should have been thrown into the scrap heap 
long ago. He says he cannot afford to replace them with 
new projectors. Let us examine into the matter. 

Two new projectors of late type would cost him let us 
say $1,200 for the sake of easy figuring. Let us further 
assume that the new projectors will last three years, at the 
2nd of which time they will be utterly useless. In other 
words, that John Jones, after using the new projectors three 
years, is going to throw them into the scrap heap as having 
no value at all. Let us also allow 8 per cent, interest on 
the investment. This means that John Jones is going to in- 
vest $1,200 in projectors, which at the end of three years 
will be entirely worn out. He will also lose the interest on 
that sum for three years, which at 8 per cent, amounts to 
$288. John Jones therefore stands to "lose" $1,488 in three 
years if he buys two new projectors. This means that in 


1,095 days 148,800 cents will have been "used up." This is 
an average of about 135 cents a day, or the value of less 
than seven admissions at 20 cents each. 

Does that exhibitor live who honestly believes that 
two new projectors will not produce a sufficiently better 
screen result than his worn and more or less out-of-date 
projectors to increase the patronage of the theatre which 
has 2,800 seats (or even a much less number) to sell every 
day by decidedly more than six additonal admissions? 

We thus see that, as a plain matter of common sense, 
John Jones CAN afford to put in new projectors; also that 
he is actually losing money every day he delays doing it. 

The projection room should have an ample supply of 
carbons and all those various repair parts and other things 
necessary to the keeping of the equipment in first class 
condition. It is impossible to enumerate the various things, 
because they will, in the very nature of things, vary con- 
siderably in different projection rooms. 

The things we seek to do in the matter of projection room 
supplies is not so much to supply a list, which may or may 
not fit the individual requirement, but to impress upon the 
mind of the theatre management the fact that "scrimping" in 
projection room supplies is, in the long run, not true 
economy, and may actually be a great source of waste. 

Safe Corporation, Baltimore, Md., is putting out a film 
storage cabinet which we regard as being pretty nearly ideal. 
In fact we very much doubt if the general design of this 
particular piece of projection room equipment will ever be 
very much improved upon, though that prophecy does not 
extend to structural details. The cabinet seems to combine 
about every desirable feature, in that it is fireproof as a 
whole and as applies to individual reel containers, is hand- 
some in appearance, rigid in construction, very elastic as to 

In Fig. 90e we have a view of the cabinet, which is named 
the Safe-T-First Film Cabinet, with the notation that in its 
final design each section holds five (5) reels, whereas the one 
shown accommodates three reels to the section. As it now is 
the 5-reel section is the minimum. The 3-reel section is no 
longer made. 

The cabinet is of all steel, insulated construction. In ap- 
pearance it exactly resembles the rather handsome cabinet 
document files seen in many offices. Each handle is attached 



to and operates the door of a single reel compartment, which 
same is fire insulated from every other single reel compart- 
ment. Each of these single reel compartments is, or may be, 
connected either with the open air or with the projection 
room vent flue, through the 
cabinet vent pipe. In other 
words, the vent pipe top, or 
cone, seen at the top of 
the cabinet in Fig. 90e, 
connects directly w,ith 
every one of the single reel 
compartments of the cabi- 
net, hence should a reel in 
any one of the compart- 
ments catch fire, it would, 
due to the very limited air 
supply, burn very slowly, 
all the smoke being con- 
veyed directly away and 
out of doors. This feature 
alone we regard as of dis- 
tinct value. 

In Fig. 90f we see one 
of the sections being lifted 
away. Remembering that 
the Safe-T-First cabinet 
sections may not be had in 
less than 5-reel capacity, it 
is seen that any desired 
capacity may be had in a 
single cabinet by piling as 
many sections as desired, 
one upon another ; each 
compartment connecting 
with the vent flues of the 

other sections, as shown in Fig. 90f the flues of the upper 
section being capped by the cone shown in Fig. 90e. 

INDIVIDUAL LOCKS.— Each individual reel compartment 
may be provided with a lock, if so ordered; also each com- 
partment is self closing. By opening the compartment the 
reel is automatically moved to the position shown in 
Fig. 90e. 

You cannot leave a compartment open, unless you de- 

Figure 90e. 



liberately block it open, because it closes automatically by 
its own weight. 

This cabinet has the hearty indorsement of the author of 
this book and of the Projection Department of the Moving 
Picture World. They are well worth their price. 

THE REWINDER.— The rewinder may be located either 

in the projection room or 
in a room immediately ad- 
joining. It will pay the 
projectionist to give a lit- 
tle thought to the arrange- 
ment of his rewinding 
table. In the first place 
it is of huge importance 
that the two elements of 
the rewinder be in per- 
fect line with each other, 
since otherwise the edge 
of the film will rub on the 
edge of the reels during 
the whole process of re- 
winding, with resultant 
weakening of the film 
track, especially if the 
sides of the reel be bent 
or crooked. 

We cannot emphasize 
too strongly the import- 
ance of carefully lining 
the two elements of the 
rewinder. Damage aggre- 
gating thousands of dol- 
lars a day is done to film 
by rewinders because of 
the two elements being out 
of line with each other. 

The two elements of the 
rewinder should be placed 
a convenient distance apart, carefully lined with each other 
and fixed firmly and permanently in position so that they 
cannot possibly get out of line. Between the two elements 
of the rewinder a hole about 3 inches wide by 4 inches long 
should be cut in the rewinder table. This hole should be 
covered with a piece of thick glass, the top of which should 

Figure 90f 


be thoroughly ground by rubbing it with fine sandpaper, or 
with fine sand under a piece of flat stone or iron. Under 
this glass install a small incandescent globe, and Just back 
of it cut a hole in the table to receive the cement bottle. 
You thus have everything convenient for making splices. 

The rewinder should in all cases be motor driven and 

The speed should be geared down by means of suitable 
pulley wheels or gears until at least 8 minutes is consumed 
in rewinding 1,000 feet of film. 

Rewinding film at high speed is bad practice. If the re- 
winding be done at the rate of 8 minutes to the thousand 
feet (10 is better) the film will be rewound in ample time 
to make room for the next reel, in theatres where sane 
projection methods are practiced, and there will be no 
necessity to watch the process of rewinding, unless there 
are repairs to make. There should be an arrangement which 
will automatically stop the rewinder motor when the pro- 
cess of rewinding is finished. This may be done in any one 
of a dozen ways, all of which have from time to time been 
described in the projection department of the Moving 
Picture World. Where the motor rewind is used a brake 
should be provided for the reel from which the film is 
being rewound, and there should be just sufficient tension 
to cause the film to be rewound snugly. 

Pulling down of film (holding one reel stationery and re- 
volving the other to tighten the film roll thereon) is pro- 
ductive of enormous damage to film in the form of scratches 
made as the layers of the film roll slip on each other. 
These scratches later fill up with dirt and form the rain 
marks with which we are all so familiar. If rewinding be 
done by motor, and be done slowly, with plenty of tension 
on the reel from which the film is being rewound, there will 
be no necessity for "pulling down," and thus much damage 
to film will be avoided. 

Another argument in favor of the slow motor rewind is 
that where rewinding must be done by the projectionist, if 
the speed of rewinding be slow and the rewind be motor 
driven, there is no necessity for the projectionist watching 
the process. He is therefore free to attend to his other 
duties, and if the rewinder be so arranged that the motor 
will automatically stop when the process of rewinding: is 
finished, the only attention the projectionist need give the 
process of rewinding is to take off the rewound reel, put it 
in the storage case, put on the reel to be rewound and 


start the rewinder going. This, of course, being true only 
when there are no repairs to be made. 

For the purpose of making repairs it is, perhaps, best to 
install a separate hand rewinder. In this connection, we 
would recommend the installation as a part of the pro- 
jection room equipment of the film splicer. 

Good film splicers may be had from any supply dealer. There 
are several good ones on the market. It is up to you to select 

AMMETER AND VOLT METER.— In the judgment of 
the author it is an exceedingly good investment to provide 
the projection room with an ammeter and volt meter, par- 
ticularly the former, and to locate them in such position that 
they will be constantly in view of the projectionist when he 
is in working position at the projector. The ammeter 
should be connected to the projection circuits in such way 
that it may be used to indicate the amperage of either arc. 

There is a certain point at which the projection arc will 
produce maximum illumination with a minimum of current 
consumed. Just a slight movement of the carbons away 
from this position will raise the current consumption any- 
where from 5 to 20 per cent., without in the slightest degree 
increasing the screen brilliancy — in fact it is likely to de- 
crease it. With an ammeter placed directly in view of the 
projectionist he is able to, and if he is a careful man will, 
maintain his arc at the point of maximum brilliancy with a 
minimum current consumption. We are firmly convinced 
that in the average theatre a projection room ammeter, if 
properly located, will pay for itself in a very short time. A 
good ammeter and volt meter may be had from your supply 
dealer. The method of connecting an ammeter and volt meter 
is shown in Fig 91, page 335. 

type projectors anchoring is not of such prime importance, 
since the weight of the machine itself is sufficient to hold 
it steady, and pedestal projectors are designed to be bolted 
to the floor. Anchor bolts may be set in a cement floor by 
drilling holes about 3 inches deep in the cement, setting 
the bolt head down in the hole, and pouring the hole full of 
melted solder. With the lighter machines, however, many 
of which are still in use, the anchoring of the machine to the 
floor is important, since the slightest vibration or move- 


merit, of the projector will produce unpleasant results upon 
the screen. The old style tables may be anchored down by 
the use of screw eyes set in the floor, wires attached to 
the machine table and to the screw eyes through a "turn 
buckle" such as may be had from any hardware store. We 
do not believe a detailed description is necessary, because 
the importance of anchoring a light machine is self-evident, 
and certainly any man fit to be a projectionist would have 
sufficient ingenuity to find a method of securing the de- 
sired result. 

TOOLS. — The projectionist should be in possession of a 
kit of tools enabling him not only to do any work incident 

Figure 91. 

to projection, but small repair jobs as well. Such a kit 
will cost quite a bit of money, but it is a good investment. 
The manager is likely to have a great deal more respect for 
the projectionist who owns a good kit of tools than for one 
who owns a ten cent screw-driver and a pair of pliers. 

In the third edition of the handbook we gave a list of 
tools, to which we see no reason for either adding or sub- 
tracting, except to remark that the management should 
provide a small hand bellows, particularly if a motor gene- 
rator set to be used. This tool should be a part of the 
projection room equipment. It is used for the purpose of 
blowing out dust and dirt from around the armature and 
pole pieces from the motor generators. The following is 
the list of tools: 

One pair "button" pliers 8 or 10 inch : one pair 8 or 10 inch 
lineman's side cutting pliers (we leave the matter of size 
open, as some prefer one and some the other) : one pair 8 
or 10 inch gas pliers: one large and one medium screw- 
driver; one screw T -driver with good length of carefully 


tempered blade for small machine screws, to be heavily 
magnetized so as to hold small screws; one pair of pliers 
for notching film, see page 288, one small riveting hammer; 
one carpenter's claw hammer; one small cold chisel; one 
medium-sized punch; one very small punch for star and cam 
pins ; one small pair tinner's snips ; pair blunt-nose film shears 
(such as clerks use) ; one small gasoline torch for soldering 
wire joints; one hack-saw. With this kit you will be able 
to do almost any ordinary job, but you will have use for 
them all. In addition to the # above the house manager should 
furnish one 8 and one 10 inch flat file, one % round file, 
one 8 inch ''rat tail" file, a small bench vise with anvil and 
some soldering flux and solder wire, and a film splicer. 

Of course many projectionists will wish to add to this kit, 
but what we have named will serve very well, taken in con- 
junction with the files and other things furnished by the 

OPERATION. — Unscrew lower end cap and fill with 
alcohol. Replace lower cap and remove upper end cap. 
Blow through hose and adjust blow-hole to proper height 
as per instructions accompanying torch. That is all there is 
to it. The torch is NOT good for any but very small jobs, 
and for wire joints. It should, except for renewal of the 
rubber hose (cost about 10 cents), and the wick (probably 
5 cents) perhaps once a year, last for years, provided you 
don't allow it to get smashed. We recommend it to you for 
the purposes named. 

TOOLS IN ORDER.— It is of the utmost importance that 
the projectionist's tools, be they many or few, be kept in 
order, neatly arranged on the wall, the screw-drivers and 
pliers within handy reach. One of the most reprehensible 
habits possible is that of dropping tools when one has 
finished using them and letting them lie until needed again. 

It would be hard to estimate how many thousands of 
times moving picture theatre audiences have sat in the dark, 
waiting patiently while a projectionist searched for the 
pliers, screw-driver, or other tool needed to make a repair, 
which he had thrown down wherever he happened to use it 
last. Often I have gone into projection rooms and found the 
tools lying on the floor in a jumbled pile underneath the 
projector. This kind of thing is not only exceedingly un- 
workmanlike, but also is decidedly sloppy. The man who 
does things that way is unlikely to make any large success, 
either of projecting or anything else. 

My advice to the projectionist is have a good kit of tools 
and keep them neatly arranged and in perfect order. 

My advice to the manager is to discharge the projectionist 
who is satisfied to own only a pair of pliers and a screw-driver, 
oi* who, having other tools, does not keep them in order. 


If he is unworkmanlike in so important an item, it is likely 
he will be unworkmanlike in other things which will reflect 
directly on the screen in the shape of faulty projection. 

say a word about cleanliness. I have suggested that tools 
should be kept in perfect order — a place for everything and 
everything in its place. 

There is nothing so tends to impress the visitor with the 
incompetency or shiftlessness of the man or men in charge 
as to see tools strewn around and (or) floor and walls cov- 
ered with dirt. 

The moment I see a slovenly, disordered, dirty room I 
form the idea that the man in charge probably is lacking in 
knowledge, and certainly lacks sufficient energy to apply 
that knowledge he may be in possession of. 

At the risk of seeming to waste space, I cannot refrain 
from the effort to impress upon you the fact that dirt and 
slovenliness will tend to lower you in the eyes of the compe- 
tent theatre manager, regardless of every other thing which 
may merit his commendation. 

The real projectionist takes real pride in his work. He 
seeks to do every part of it efficiently and well. The little 
extra work necessary to keep things clean and in order are 
as nothing, compared with the disgrace of tolerating disorderly 
dirty surroundings. 

The projection room should be swept thoroughly once every 
day, and once each week should be cleaned thoroughly in its 
every part. 

ANNOUNCEMENT SLIDES.— It is frequently necessary 
to make announcements to the audience. There are a great 
many different ways in which very good appearing slides 
can be quickly prepared. There are inks on the market, in 
several colors, with which one may write, using an ordinary 
pen, on clean, plain glass, just the same as he could write 
on paper. There are also a number of slide coatings for 
sale on which writing may be done with a pointed instru- 
ment. These slide coatings are particularly to be desired 
for any slide which must be made on the spur of the 
moment, by reason of the fact that a number of them can 
be gotten ready and laid up on a shelf in a pile, where they 
will keep indefinitely. If anything happens and you wish to 
say something to the audience, the projectionist can write 
on these slides with anything having a sharp point. For 
instance, suppose something occurs that will cause a delay 
of two minutes. Within five seconds the projectionist can 


write on one of these slides "Unavoidable Delay of Two 
Minutes," stick it in the stereopticon and project it to the 
screen. The audience will then be satisfied to wait for that 
length of time. I only suggest this as one possible way in 
which slides of this kind may be utilized. They should be 
kept in the projection room ready for instant use. Please 
understand in this I am not referring to program slides 
which the manager himself will wish to prepare, but merely 
these designed to be used for emergencies. 

projection room is a matter which should be carefully 
planned before the construction of the room is begun, es- 
pecially if the walls in which the conduit is imbedded are 
to be of concrete or brick. 

The projection room service wires must, of course, be 
large enough to carry the entire load of the projection room 
without overload, and with only about 2 per cent, 
voltage drop. By this we mean that if there are, for in- 
stance, two projection lamps, a dissolving stereopticon, a 
spot light and four incandescent lamps; the projection 
room feed wires must be large enough to carry the com- 
bined amperage of all these lamps when they are all burn- 
ing; though they need only operate at 2 per cent, voltage 
drop at the normal load used. It is quite true they are not 
likely to ever be all in use at one time, but this doesn't alter 
the fact that the correct procedure is to make the wires 
large enough to supply them all without overload. 

The necessary amperage capacity of the projection room 
feed wires is computed as follows : First estimate the com- 
bined amperage. Suppose there are two projectors and you 
propose using 60 amperes at the arc of each through re- 
sistance. This would call for 60 + 60 = 120 amperes at the 
two projector arcs. The dissolving stereopticon will, if one 
be used, probably require a total of 30 amperes for the two 
lamps, and if there be a spot light 25 amperes will probably 
serve for it. We will assume the incandescents to require 
five amperes. We thus have 120 + 30 + 25 + 5 = 180 
amperes. Turning to table 1, Page 70, we find that if the 
service circuit be 2-wire it will be necessary to install 
No. 000 wires to carry that amount of current without over- 
load, modified by the fact that length of circuit forms an 
element which must be reckoned with. See "Figured Volt- 
age Drop," Page 74. 

If the feeders are 3-wire, then, since when the lamps are 


all in use one-half would burn in scries with the other half; 
providing the balance be perfect (see 3-wire system, Page 
85), the amperage requirement would thus be cut in half 
and No. 2 wires would serve, again provided length of 
circuit be not too great. 

If the projection room service circuit be 3-wire, but it be 
required that the projection arcs be connected across the two 
outside wires on 220 volts, then it would be necessary that 
the two outside wires have the same capacity as though it 
were a 220 volt 2-wire system. 

However, regardless of what the condition may be, the 
projectionist should figure the voltage drop as per the 
formulas laid down on Page 74, since voltage drop due 
to too high resistance of the projection room service wires 
will be registered on the meter, and must be paid for just 
the same as though the current were consumed at the arcs, 
modified when current is taken through adjustable resist- 
ance. In that case the excess resistance in the wires may be 
compensated for by using less resistance in the rheostat, 
but the condition, nevertheless, is a bad one. 

If all the lamps be operated from motor geenrator set, 
rotary converter or motor arc rectifier, or through a low 
voltage transformer (economizer, compensarc, inductor, etc.) 
then the projection room service wires need only be 
large enough to supply the primary capacity of these de- 
vices, always assuming the aforesaid motor generators, 
rectifiers or transformers are to be supplied by the pro- 
jection room service circuit. If they be locited in the base- 
ment or elsewhere and the projection room be supplied by 
wires from them, then, of course, since these wires will 
carry the secondary current, their size must be figured on 
that basis. 

In Fig. 92 the general layout of a projection room switch- 
board is shown. There should be a main switch (A) sup- 
plied with link fuses (B-B-B). This switch and the fuses 
carry the entire projection room load. The incandescent 
circuits should be taken through a separate circuit con- 
trolled by a separate knife-switch as per C. Cut out blocks 
D-E-F (there may be as many as are required), carry fuses 
for the various circuits and, if desired, switches also. In 
Fig. 92 we will assume circuits 1 and 2 supply the projector 
arcs, circuits 3 and 4 the dissolving stereo circuit and cir- 
cuit 5 the spotlight. This layout is only designed to be sug- 
gestive. It shows how a very acceptable projection room 


Figure 92. 


switchboard may he built up from switches and fuses 
mounted on a suitable insulating base. The board shown in 
Fig. 92 is built of Y% inch asbestos mill board and is en- 
closed in a metal cabinet similar to the one shown in Fig. 
17, Page 104, only larger. This type projection room 
switchboard may easily be constructed by using slate-base 
knife-switches equipped with fuses, if it is so desired ; and 
it is quite practical to use link fuses instead of cartridge 
fuses. In fact in some cities this is required by law for 
projection circuits. 

BALANCING THE LOAD.— Where a 3-wire service cir- 
cuit supplies the projection room, and it is permitted tha' 
connection be made to the neutral, the power company 
should in all cases be consulted as to how they would prefer 
to have the projection arcs connected. It is, of course, im- 
possible to balance the projection room load on a 3-wire 
system, because ordinarily only one projection arc will be 
burning at a time. This, however, does not apply to the 
dissolver, the arcs of which should always be connected to 
opposite sides of the system, since when the dissolver is 
used its load will be perfectly balanced between the two 
sides. If both dissolver lamps were connected on one side, 
when the dissolver is in use, it would have an unbalancing 
effect of whatever the capacity of the two dissolver lamps 
may be. Ordinarily it would be undesirable to connect both 
projection lamps to one side, particularly if you are using 
high amperage, because when the arc of the idle projector 
is struck to heat the carbons before it is time for a change 
over, for a short time the entire load of both projectors 
would be on one side of the system, which would mean an 
unbalancing effect of anywhere from 60 to 200 amperes (or in 
some cases even more) which would be sufficient to be ob- 
jectionable even on a system, supplied by large generators. 

As a rule power companies will always want projection 
lamps connected on opposite sides, but as we said in the be- 
ginning it is good practice to consult them in a matter of 
this kind. In the foregoing we are assuming the arc lamps 
to be taking current directly from the mains .through re- 

MIZERS, ETCETERA.— Where current is taken through a 
motor generator set, and the supply lines are 3-wire, we 
would recommend the purchase of apparatus having a 
motor of the voltage of the two outside wires. This not only 


avoids any unbalancing effect, but has the additional ad- 
vantage that high voltage motors work more efficiently than 
the low voltage machines. 

Where current is taken through a mercury-arc rectifier, 
and the projection room is supplied by a 3-wire system, we 
would recommend that the rectifier be connected across the 
two outside wires, which may be done merely by making the 
proper internal connections in the rectifier itself. See 
Page 567. 

Where projection current is taken through low voltage 
transformers (economizers, inductors, compensarc, etc.) and 
the projection room feeders are 3-wire, it is better to pur- 
chase an economizer to operate on a voltage of the two 
outside wires. Such an economizer is just as efficient as 
the one of lower voltage, and the unbalancing effect is thus 

The location of the projection room switchboard cabinet 
will necessarily be determined by local conditions, but it 
should be remembered that inconveniently located appa- 
ratus invariably tends to decrease efficiency of operation. 

There is nothing to be gained by making things incon- 
venient for the projectionist. On the other hand there is 
much to be lost by so doing. 

PROJECTION ROOM FUSES.— We would suggest that 
the projection room service fuses be placed as shown in 
Fig. 92, rather than on the other side of the main switch. 
Inasmuch as the projection room feeder circuit, including 
the main switch, is protected by fuses in the main house 
switchboard or elsewhere, there is no necessity for pro- 
tecting the main projection room switch further; and it is 
more convenient to install fuses at B if the fuse block be 
"dead" than if it be alive, as it would be if the fuses were 
on the other side of the switch. 

power companies will not permit the neutral wire of the 
3-wire system being run to the projection room. This com- 
pels the use of 220 volts, which, if rheostats be used, is very 
wasteful indeed. The reason for the refusal to allow the 
neutral to be run to the projection room is the heavy un- 
balancing effect of the projection arcs, as already ex- 
plained. It is quite possible that this unbalancing effect 
may be very serious, from the power company's viewpoint, 
especially in a small city where there are a number of mov- 
ing picture theatres and the power company's generators 



likely to be pretty heavily loaded. Suppose, for instance, 
a 3-wire street main supplies five moving picture theatres 
each of which have two projectors, all connected to the same 
"side" of the system. It might very easily happen that the 
projectionists of all five theatres would chance to be chang 
ing from one machine to the other at the same time, and 
that all struck the arcs of their idle projector at approxi- 
mately the same time. This would mean, assuming they 
were pulling 60 amperes at each arc, a total unbalancing 
effect of 600 amperes, since while the arcs of both pro- 
jectors were burning each theatre would be using 12') 



Ware* /*/*>*. 

7~*T&T 1**10* *T-T*WCO 

&C**9*j/*e*rt.y To **"**. i.. 



7Tj T-tr*T 

Figure 93. 

amperes at its projection arcs. This would probably have 
the effect of very greatly overloading the generator attached 
to that side of that particular street main, at least tempo- 
rarily. Even if these five theatres each had their two pro- 
jection arcs on opposite sides, when only one arc was 
burning in all of them it would mean an unbalancing effect 
effect of 60 x 5 = 300 amperes, so that you see the light 
company is, from their viewpoint, perfectly justified in 
demanding that only the outside wires of their 3-wire system 
be used. 

This does not, however, hold good if current be taken 
through rheostats. 

In that event the compelling of theatres to connect to the 


two outside wires would simply mean that instead of over- 
loading one generator by a given amount, both generators 
attached to the system would be overloaded that amount. 
Instead of one generator pulling an unbalanced load of 600 
amperes, as before set forth, if connected to the two outside 
wires each generator would have to pull a load equal to 
600 amperes at 110 volts when all arcs are burning, there- 
fore the only result would be a big additonial (double) load 
on the power plant, and an entirely useless waste of elec- 
trical energy. 

The reason for this is that when taking current through 
rheostats 600 amperes at 110 volts equals 66,000 watts, but 
600 amperes at 220 volts equals 132,000 watts, the extra 
energy being consumed in the resistance itself. 

While the light company has the undoubted right to de- 
mand that the projection lamps be connected to opposite 
sides of a 3-wire system when current is taken through 
economizers, it has no right to demand that the projection 
lamps be connected to the outside wires of a 3-wire system 
if current is taken through rheostats, since it gains abso- 
lutely nothing by that sort of procedure except the sale of 
double the amount of electrical energy. 

Where projection current is taken from a 3-wire system 
through either a motor generator rotary converter, mer- 
cury arc rectifier or economizer, the power company is en- 
tirely within its rights in demanding that the theatre use 
the two outside wires only for projection current. 

tion room lighting presents a very distinct optical problem, 
though that fact seems to be seldom realized by either the 
projectionist or the theatre management, and apparently is 
seldom realized or recognized by the architect. It is an 
optical impossibility to have a clear sharp view of a screen 
located perhaps 100 or more feet away when looking out of 
a well lighted room through a comparatively small opening 
in its wall. This is true under any circumstance, but is 
especially true if the walls surrounding the opening (ob- 
servation port) be light in color, and since it is impossible 
that the projectionist judge of the fineness of focus unless 
he has a clear sharp view of the screen, it follows that 
sharpness of focus or definition will inevitably suffer if the 
observation port be small, and the projection room unintel- 
ligently lighted. 

After visiting hundreds upon hundreds of theatres in all 


parts of the country the writer has never yet seen but one 
in which a really good view of the screen was had from a 
well lighted projection room. This single exception is 
described on Page 311, but since much time will be required 
to educate theatre managements and public officials to the 
large projection room observation port, we must in the in- 
terim deal with conditions as they are. The average pro- 
jection room has a relatively small observation port, and 
some of them have a very small one. It is difficult to get a 
sharp view of the screen through such a port under any 
condition, and it may be stated as a matter of fact that un- 
less projection rooms having these ports are kept dark the 
screen result will inevitably suffer. 

In such rooms we would recommend that where the ceiling 
is high — say not less than 10 feet — a row of lights, say four 
in number, be placed at the ceiling, and as close as possible 
to the front wall, and then that a board or shelf be run 
along the entire length of the front wall extending out just 
far enough to prevent the light from these lamps striking 
the rear wall within 6 feet of the floor, the space above the 
shelf to be painted white. 

This suggestion if properly carried out will set up as 
good an optical condition as could be expected, but it will 
not work if the ceiling is low, because in that event the 
light would shine directly into the projectionist's eyes. 

With a low ceiling the lights may be placed at will, and 
switches provided so that the projectionist may turn the 
lights out when projecting. 

Another excellent plan for projection room illumination 
is that used by the Cinematograph Theatres, Ltd., England, 
It consists of inverted bowl indirect lighting fixtures in 
which two distinct circuits are installed, one carrying suffi« 
cient lamps to illuminate the room very dimly, just enough 
to enable the projectionist to find his way about, the other 
circuit serving to give brilliant illumination. It is forbidden 
that the "bright" circuit be used except in emergency. 

We would suggest that unless some such plan be adopted 
the management of theatres absolutely forbid the burning 
of any incandescents in the projection room when the pic- 
ture is running, except in case of emergency or while 
threading the idle machine. The management has a peri 
feet right to make this rule, because upon its observance 
depends, to a considerable extent, the continuous excellence 
of screen results. 


GROUND WIRE.— It is highly desirable that a perma- 
nent, known ground be established in the projection room, 
and this may be best done by attaching a No. 22 or largci 
copper wire to a water pipe, or else soldering the end of 
such a wire to a copper plate not less than one foot square, 
and burying the plate, imbedded in powdered coke, in the 
ground deep enough to secure a permanent contact with 
moist earth. If the wire is attached to a water pipe the 
pipe should be scraped with a file until it is bright, the 
wire thoroughly cleaned and wrapped around the pipe tight- 
ly several times. It will not be practical to solder anything 
to a water pipe if the pipe contains cold water. Another 
and even better plan is to file the pipe clean and then make 
a band, either of brass or copper, and clamp the same to 
the pipe by means of small stove bolts, first having soldered 
the end of the ground wire to the band. Having attached 
the ground wire, either to the water pipe or to the buried 
copper plate, it should be carried to a convenient point in 
the projection room, and the end of it attached to one bind- 
ing post of an ordinary incandescent lamp socket, as shown 
in Fig. 93. We then attach another copper wire to the other 
binding post of the lamp socket, this latter wire being long 
enough to reach any part of the apparatus it may be desired 
to test. A good place for the lamp socket is in the ceiling 
immediately above the projectors, unless the ceiling be too 
high, in which case it may be attached to the front wall 
between the two projectors. Having established an ordi- 
nary incandescent lamp in the socket, testing for grounds 
becomes a very simple matter indeed, since we have only to 
touch the thing it is desired to test with the raw end of the 
test wire (see "Testing for Grounds," Page 356.) 

We are indebted to John Auerbach, New York City, for a 
most excellent testing installation by means of which it is 
only necessary to close a switch in order to test for grounds 
in either carbon arm, or to test the fuses even though they 
be located at a distant point as, for instance, in the base- 

The following is the key to the diagram shown in Fig. 94 : 

(A) Incandescent lamp. 

(B) Incandescent lamp. 

(C) Single Pole Snap Switch, 

(D) Single Pole Snap Switch. 

(E) Single Pole Snap Switch. 

(F) Single Pole Snap Switch. 



(G) Wire leading to binding post of positive carbon arm. 

(H) Wire leading to binding post of negative carbon arm. 

(I) Edison 3-wire system. 

(J) Incandescent cntout in projection room. 

(K) Projection circuit cutout in projection room. 

(L) Positive fuse of projection circuit. 

(M) Negative fuse of projection circuit. 

That part of the diagram drawn in heavy lines indicates 
wiring to test for grounds in either carbon arm. That part 
drawn in light lines indicates the wiring to test fuses in 
main service fuses. The device therefore may be wired 

+ 1 1 

H G 



— 1* 



1 if 

Mr l 



s , 

-H>+ T" 



Figure 94. 

for either or both of these purposes. When the tester is not 
in use all switches should be open. 

(A) To test for ground in positive carbon arm close 
switch C. If lamp lights, a ground is indicated in that arm. 
If lamp does not light there is no ground. 

(B) To test for ground in negative carbon arm close 
switch D. If lamp lights, a ground is indicated in negative 
carbon arm; otherwise not. 

(C) To test positive projection room service fuse close 
switch E. If lamp lights fuse is O. K. ; otherwise it is not. 

(D) To test negative projection room service fuse close 
switch F. If lamp lights fuse is 0. K. ; otherwise it is not. 

Weiring connections to projection cutout should be made 
ahead of the switch and the switch should always be open 


when tests are being made. The projectors should be per- 
manently grounded by a ground wire and this ground wire 
should be left connected during the making of the tests. 

TROUBLE LAMP.— A "trouble lamp" should of course 
be installed, and the best way to do this is to place in some 
convenient location a permanent socket containing a plug 
to which sufficient cord is attached to reach any part of the 
room. At the extremity of this cord should be a lamp 
socket and an incandescent lamp of suitable power, covered 
by a wire guard to prevent breakage. 

PROJECTOR CIRCUITS.— As has already been said, 
Page 302, the projector circuits should be carried under the 
floor to a point immediately under each projector lamp 
house, but where circumstances for any reason prevent this, 
or make it difficult, then the projector circuit may be car- 
ried above the ceiling, or if that is impractical, then the 
projector circuit may be carried along the ceiling to a point 
just to the rear of the lamp house, whence the conduit may 
drop down to a point just above the rear end of the lamp 
house. There is in fact very little real objection to this 
latter plan, provided the lamp house be piped to the vent 
flue as per Page 363, so that the wires will not be subjected to 
the heat arising from it. 

SWITCH ENCLOSURE.— It is usually required that all 
switches (except those of the enclosed type) and fuses be 
enclosed in a metal cabinet. This requirement unquestion- 
ably adds an element of safety, since there is always the 
chance of something falling against an unprotected switch 
or open fuse contacts, and causing trouble ; also there is 
always the possibility of the projectionist himself acciden- 
tally coming into contact with unprotected switches or fuse 
contacts and receiving a disagreeable shock, or even a burn. 

The projector switch itself must be of the "enclosed" 
type, i. e. enclosed in a sheet metal casing. 

T OUBLE THROW CONNECTION.— It is very bad prac- 
tice io connect the two projectors through a double-throw 
switch to the center contacts of which the supply is at- 
tached, so that it is necessary to extinguish one projector 
lamp before the other can be lighted. This sort of connec- 
tion is only permissible in cases where current is taken 
through a single motor generator or rectifier of such small 
capacity that it cannot supply both lamps, even for a lim- 
ited time. Even under this condition it is very much better 



to wire in parallel (multiple) and "steal" the current from 
one lamp to the other, than to adopt the above described 

Where the motor generator or mercury arc rectifier is too 
small to allow burning both arcs together for even a limited 
time, or where only one economizer is used, Fig. 95 offers 
an excellent plan by means of which the idle lamp may be 
warmed and a crater burned in by means of a rheostat. By 
tracing the connections in Fig. 95 it will be found that with 
the switch in the position shown in the diagram the right 







Figure 95. 

hand lamp is taking current through the compensarc while 
the left hand lamp is taking current directly from the sup- 
ply lines through the rheostat. When the four-pole switch 
is thrown over the same condition will prevail, except that 
the left hand lamp will then be taking current through the 
compensarc and the right hand lamp through the rheostat 
The arrangement is a most excellent one under the condi- 
tions we have named, and the wiring of the diagram is 
sufficiently plain that any projectionist or electrician should 
be able to make the installation without trouble, substi- 



tuting either a motor generator or a mercury arc rectifier 
for the compensarc, if either one of them be used. 

POLARITY CHANGER.— Where the supply is taken 
from a small D. C. plant it sometimes occurs that when 
dynamos are changed the polarity changes, which requires 
the instant switching of your own wires to bring the posi- 
tive back to the upper carbon. This may quickly be ac- 
complished by the installation of a double-throw double- 
pole switch, such as is seen in Fig. 96. Throwing this switch 

Figure 96. 

over changes the polarity of the wires. The cross wires 
should be protected by flexible insulating tubing in addition 
to their own insulation. 

Fig. 97 is the diagrammatic representation of a combined 
polarity switch and fuse changer. By throwing switch A a 
new set of fuses is brought into use and by throwing switch 
B the polarity at the arc is changed. 


various reasons it is frequently desirable to make connec- 
tion to two separate sources of electrical supply. One may 
have one's own light plant, but wish, in case of accident, to 
be able to instantly connect to the wires of the city plant. 
This may be done, but details may vary widely in different 
cases. Suppose, for instance, we have a house plant deliver- 
ing direct current at 110 volts, while the city plant produces 
A. C. at 110 volts; both systems two-wire. The problem 
then is simple. 

Install a double-pole, double-throw switch, as per Fig. 98. 
The house plant being D-C, we shall not need nearly so 
much amperage from it as would be necessary for equal 
screen illumination with the city plant, A-C; therefore, we 
install two rheostats, A and C, the lower one, A, to be used 



with the D-C house plant. B is a double-pole single-throw 
knife switch which is open when D-C is in use, so as to use 
only rheostat A. When we change to A-C, however, we 
close switch B, thus cutting rheostat C in multiple with 
rheostat A. Rheostat C should have capacity sufficient to 
build the combined amperage of the two up to that neces- 
sary for good illumination of the screen. Suppose we us& 
35 amperes D-C. In order to secure anything like the same 
curtain brilliancy rheostat C must have capacity sufficient 
to deliver 25 amperes which, combined with the capacity of 
rheostat A, will give 60 amperes at the arc. But we must 
remember that, owing to the shorter A-C arc, hence the less 
arc resistance, rheostat A will deliver somewhat more cur- 
rent on A-C than it will on D-C, the voltage of the supply 
being the same in both cases. We will probably, therefore, 
be not far out of the way if we have rheostat C of capacity 
to deliver 20 amperes at the arc. 

Figure 97. 

We may, however, instead of this, install a transformer 
(economizer, inductor, compensarc, etc.), in place of rheo- 
stat C, Fig. 98, and with a triple-pole double-throw switch, 
wired as per Fig. 99, cut out resistance A, Fig. 98, substitut- 
ing the economizer therefor. Merely throwing the switch 
over would then change from rheostat to transformer, and 
vice versa, though the transformer would be "alive" in the 
sense that you could get a shock from it. But this would 
do no harm. If you wish to "kill" the transformer entirely 
when using the rheostat, it may be done by installing a 
S. P. S. T. switch at X, Fig. 99. 



Please understand there are many other switch arrange- 
ments possible. Such things may be done in many ways. 
Those suggested merely illustrate two possible methods. 
Another and still better way to cut the two rheostats in 
multiple, Fig. 100, is by means of a triple-pole, double-throw 

A careful tracing out of the connections in Fig. 100 will 




e A — — 



-0 /? qy 


Figure 98. 


show that when the switch is thrown to the A-C supply 
side the two rheostats are in multiple, while when the D-C 
side is in use only rheostat 1 is working. Should the supply 
voltage be higher on one system than on the other, a higher 
voltage rheostat could be substituted for A, Fig. 98, and 
rheostat C be made of such capacity that it will bring the 
amperage up to normal when on the lower voltage. 

GROUNDS. — Grounds are perhaps the one most puzzling 
thing to the novice. They also very often tax the knowl- 
edge of experienced projectionists. This we believe arises 
partly from the fact that the term, as used, has, strictly 
speaking, more than one meaning. In one sense, a "ground" 
means a current carrying connection with the earth which 
offers a path through the earth to a wire of opposite polar-* 
ity. When we speak of a ground in a rheostat, we how- 
ever do not necessarily mean that there is any connection 
with the earth itself. Two coils may be "grounded" to the 
frame of a rheostat in such way that a part of the resistance 
of the rheostat is eliminated, notwithstanding the fact that 
the rheostat as a whole sets on an insulating shelf, and has 



no possible connection of any kind whatsoever with the 
earth. In its simplest sense the term "grounded" means that 
one wire of a circuit has current carrying connection with 
the earth, though this does not necessarily mean that there 
will be current leakage into the earth. This latter will only 
occur when the ground offers an electric path to a wire of 
opposite polarity which is attached to the same generator. 

EDISON SYSTEM GROUNDED.— The neutral wire of 
all Edison 3-wire systems is permanently grounded to 
earth. This is a true ground, and if an accidental ground 
occurs on either of the other wires of the system there may 
be and probably will be current leakage. 

Right here let us make it clear that the rather common 
belief that current seeks to escape from the wires into the 
ground is entirely wrong, except when by so doing it can 
find a path to a wire opposite polarity which is attached to 
the same generator. See Page 6. Let us also further 
emphasize the fact that : 

Current generated by one dynamo has absolutely no affin- 
ity for the opposite polarity of another dynamo except when 

Figure 99. 

the two generators be electrically connected, as in the 3- 
wire system, in which case they are to all intents and pur- 
poses one machine. 

Fig. 101 is a diagrammatic representation of a true 
ground. A is a circuit attached to generator G. B and D 
are subsidiary circuits branching from it, and C is a circuit 
attached to another generator Y. Now let us assume a 
ground to exist at point Z in the l«wer or negative carbon 



arm of arc lamp F on circuit D, and that a ground develops 
at X on the positive of subsidiary circuit B. Let us also 
assume that a ground exists at X on the negative of circuit 
C attached to generator Y. The result of all this would be 
that although the ground at X on circuit C is considerably 
nearer the ground at X on circuit B than is the ground at 
Z, and that quite possibly it would therefore offer decidedly 

D. C. Supply 

A. C. Supply 


r-= — 


Figure 100. 

less resistance to the passage of current, the current never- 
theless pays no attention to the ground at X on circuit G 
but travels through the earth to point Z where it can enter 
a negative wire attached to its own generator. On the other 
hand should a ground develop in rheostat E, which is on the 
positive of circuit D, and a ground develop at O in circuit A 
the current would enter the earth, follow the dotted lino 
and enter the negative wire at O. 

Let it be understood, however, that it does not necessarily 
follow that because two wires of opposite polarity have 
current carrying connection with the earth there will be 
current leakage, because the ground thus established may 
have such high resistance that ordinary voltage will not 
overcome it. In reaching the earth through a ground the 
current will often follow a devious path through water or 
gas pipes or electric conduits, the latter being always 

The grounding of the 3-wire system is a puzzle to many. 
There are two kinds of 3-wire systems, viz.: The Edison 
system, in which the neutral is always thoroughly grounded, 
both at the generator and at other points along the line ; 
and the 3-wire system in which the whole system is insulat- 
ed from the earth. The latter system is only used by small 
isolated plants. 

The reason for grounding the neutral in the Edison sys- 
tem is to prevent any possibility of the conduit in buildings 



becoming charged at 220 volts, or, to put it in electrical 
terms, to limit the difference in potential between any wire 
in the conduit system in buildings to 110 volts. 

With the Edison 3-wire system, the test lamp will not 
show a light from neutral to ground because the wire is 
already grounded, hence if the carbon arm of your projector 
lamp which is attached to a neutral wire of an Edison sys- 
tem be grounded there will be no effect unless your rheostat 
be on the neutral wire, in which case the fuses may blow 
when the arc is struck. This latter is by reason of the fact 
that the striking of the arc completes the circuit through 
the ground, as indicated in Fig. 101, which might eliminate 
the rheostatic resistance, leaving only the resistance of the 
arc and such resistance as the ground may offer, which may 
be more or less than was offered by the rheostat itself. It 
might incidentally be said that theoretically it would be 
quite possible when using an Edison 3-wire system and 
rheostatic resistance to locate the resistance on the outside 
wire, remove the insulation from the carbon arm to which 
the neutral is attached, disconnect it from the wire and 

fr J 



' K. 


° /L_J \ 

* _^....-4 




Figure 101. 

thoroughly ground the carbon arm, whereupon the arc 
would operate the same as though it was connected to the 
neutral. The above, however, is not a practical thing to do 
because of the fact that any ground which might be estab- 
lished would in all human probability offer very much 
higher resistance than would be offered by the copper wire, 
also the resistance offered would probably not be constant, 
but variable and unstable. * 


TESTING FOR GROUNDS.— Grounds may be tested for in 
a number of ways. A battery and buzzer or a magneto bell 
may be used. It is even possible to test for ground with just 
a plain copper wire, depending upon the spark resultant 
upon making and breaking contact to disclose the passage 
of current; but the latter plan is not recommended, since if 
the ground be a heavy one a heavy flash and blowing of 
fuses might occur. 

The practical testing tool for the projectionist is, however, 
the test lamp, a permanent form of which is illustrated in 
Fig. 102. For two wire circuits a test lamp consists of a 
socket containing an incandescent lamp of the voltage of 
the system, with two wires attached thereto. These wires 

Figure 102. 

may be of any convenient size and length. Fig. 102 illus- 
trates a test lamp to be used either with a 2-wire or a 3- 
wire system, wires A-B being used for testing across the 
two outside wires, and wires A-C for testing from neutral 
to either outside wire. 

Taking Fig. 101 for example, and considering lamp F on 
circuit D as a projector arc lamp, if we disconnect the pro- 
jector ground wire, thus insulating the projector from the 
ground, and then touch one wire of the test lamp to the 
upper carbon and the other to the frame of the lamp back 
of the insulation of the lower carbon arm, and the test lamp 
lights ; or there is a spark when the wire is rubbed along 
the metal of the lamp frame, we know there is a ground in 
the lower carbon arm. The arc must be not burning and 
the carbons of the lamp must of course be separated when 
the test is made. 

In using the permanent test lamp described in Fig. 93 we 


would disconnect the projector ground wire and touch the 
wire of the permanent test lamp to the lamp frame. If 
there is a spark as the wire is rubbed along the metal, or 
if the test lamp lights (sometimes a ground may be existent 
but of such high resistance that there will not be sufficient 
current passing to heat the filament of the test lamp red. 
In this event the ground is detected by the spark at the end 
of the wire) we know one or the other of the carbon arms 
are grounded. If it is an Edison 3-wire system we know it 
is the arm not connected to the neutral. If it is not an 
Edison 3-wire system then we have only to disconnect one 
of the wires of the lamp to determine which arm it is. If 
there is no further evidence of a ground we know the 
trouble is in the arm from which the wire has been discon- 
nected. If the test lamp still lights, or there is still a spark 
we know the trouble is in the carbon arm which is still 

Always disconnect the ground wire of the projector be- 
fore attempting to test your lamp for grounds, except when 
using the Auerbach method. 

TESTING WITH BATTERY.— Another simple method of 
testing the arc lamp for grounds is to use a dry battery. 
No bell is necessary. Just connect two wires to the battery 
and, with the projector table switch open, touch one wire to 
the lamp frame and the other to first one and then the other 
carbon arm. If there is a ground there will be a spark, but 
this test should be made in a darkened room, because the 
spark may be faint, due to the low voltage of the battery. 
As a matter of fact the battery test is not very reliable be- 
cause a high resistance ground which might let current 
through when subjected to 110 volt pressure might not show 
at all with the battery test. 

The magneto test is of course the best of all, but the pro- 
jectionist can hardly afford to add a high-priced magneto 
to his tool kit. The magneto test is best by reason of the 
fact that the magneto produces voltage very much higher 
than that of any projection circuit. To test the arc lamp 
with a magneto you have only to open the projector table 
switch and connect one of the magneto wires to the lamp 
frame and touch the other alternately to the two carbon 
arms. If there is a ground in the insulation the bell will 

With the insulated 3-wire system the test lamp acts ex- 
actly the same as it does with the plain 2-wire system- 




locating of a grounded rheostat coil or grid is a very puz- 
zling thing to the novice, as well as to many projectionists. 
It really is, however, a very simple matter. 

In Fig. 103 we have the diagrammatic representation of a 
rheostat, in which A M C D, etc., are the coils or grids, one 
of which, E, is "grounded to the frame" at X, meaning by 
this that it has current carrying connection with the frame 
at that point. Assuming that we wish to test the rheostat, 

Figure 103. 

Fig. 103, to find out whether or not it is in good order; using 
a magneto, or a bell and battery, first touch the rheostat 
binding posts with the two leads from the bell and battery, 
or the magneto. If the bell rings it indicates that the circuit 
is complete; that is to say, no coil is broken or disconnected. 
Next touch one binding post (either one) and the outer 
casing of the frame of the rheostat. If you get no ring, 
then the rheostat may be considered as in good order, ex- 
cept that, as before indicated, there may be trouble which 
would develop when the rheostat is subjected to full voltage 
which would not be indicated by the low voltage of a bat- 
tery, but which would be discovered by a magneto, and 


except for one thing which cannot be located with a bell, a 
magneto or test lamp, viz.: two coils being sagged together, 
which would eliminate a part of the resistance of the 
rheostat without breaking the circuit. 

This latter could only be determined by a physical exam- 
ination of the rheostat, or by observation when it was in 
actual use, in which latter event the point of contact be- 
tween the two coils or grids might, and probably would 
be, heated sufficiently to become visible. 

Rheostats may be tested with a test lamp in a number of 
ways. . First, assuming the rheostat as a whole to rest upon 
insulating material, with the current on, attach your test 
lamp to the frame of the rheostat and to a wire of opposite 
polarity. If there is a spark at the point of contact, or if 
the test lamp lights, the coils or grids are grounded to the 
frame at some point, and the exact point may be located as 
hereinafter described. 

Another method would be to disconnect the wire leading 
from the rheostat binding post, connecting the same to one 
of the test lamp leads then, first having "frozen" the carbons 
of the arc lamp, touch the rheostat frame with the other 
test lamp lead. If the test lamp lights, or if there is a 
spark at the point of contact, then the coils or grids of the 
rheostat are grounded to the frame at some point. If, how- 
ever, there is no spark, or if the test lamp does not light, 
then the coils and grids are insulated from the frame. 

Still another way, again assuming the rheostat as a whole 
to rest on insulating material, is to disconnect one of the 
wires from the rheostat binding post and, with the carbons 
of the arc lamp "frozen" and the projector table switch 
closed, touch the disconnected wire end to the frame of the 
rheostat. If you get a spark there is a ground. This latter 
method of course amounts in effect the same as the one 
previously described, except where the test lamp is inter- 
posed between the wire and the frame its resistance limits 
the current flow and there is no danger of blowing fuses, 
which might occur if the last test named were applied. 

Suppose we have applied one of the before described tests 
and find there is a ground in the rheostat, indicating that 
one or more of the coils or grids has current carrying con- 
nection with the rheostat frame. How are we to discover 
the particular coil or grid at fault? That is the point which 
puzzles so many, but it is a point which becomes very 
simple when we examine it in the light of common sense. 


First disconnect the wire leading from the rheostat to the 
arc lamp, leaving only the wire connected which leads from 
the source of electrical supply to the rheostat. Now, first 
having removed the casing of the rheostat, connect one of 
your test lamp wires to the frame of the rheostat and the 
other test lamp wire to a wire of opposite polarity. Assum- 
ing we have disconnected the wire from the left-hand binu- 
ing post, in Fig. 103, we will disconnect coil or grid A, and 
if the test lamp still burns, or if there is still a spark when 
its contact with the rheostat frame is made and broken, we 
know the trouble is not in A, since the ground still exists. 
We therefore connect BCD and E. When coil or grid E 
has been disconnected the test lamp goes out or the spark 
ceases, hence we know the trouble lies in that coil or grid. 
The trouble in coil or grid E may be due to direct connec- 
tion with the frame caused by sagging, or it may be and 
probably is due to a fault in the insulation. 

If a rheostat consists of two banks of coils or grids, con- 
siderable labor can be saved by disconnecting one bank 
from the other, and then testing each as a whole to find out 
which half the ground is in. It is then only necessary to 
disconnect the individual coils or grids of the defective side. 

GROUNDING THE PROJECTOR.— It is always advisable 
that the projector lamp house, mechanism and frame be per- 
manently grounded to the metal of the projection room, if 
any there be, and then the whole may or may not be thor- 
oughly grounded permanently to a water pipe. 

The reason for grounding the projector to the projection 
room metal work is that if the projector be insulated from 
the metal of the projection room and the lamp should be- 
come grounded to the metal of the lamp house it would 
charge the whole mechanism with voltage, and, should the 
projectionist in the act of putting a reel in the magazine 
touch the reel to the magazine and the metal of the projec- 
tion room there would be a spark which might set fire to 
the film. 

There is no real necessity for the grounding of the metal 
of the projection room as a whole. It may or may not be 
done, as best suits the idea of the individual. 

EFFECT OF GROUNDING.— The effect of grounding the 
projector lamp is that current is wasted and the brilliancy 
of the light is itself likely to be affected, particularly if the 
ground be a heavy one, since a portion of the current is 
escaping through the shunt circuit produced by the ground, 


instead of passing through the carbons and producing light. 

Projectionists will do well to test their lamps for ground 
every day. It only takes a few moments and is well worth 
the trouble. 

One prolific source of current leakage in the arc lamp is 
due to carbon dust settling across the insulation of the car- 
bon jaws. This is not so likely to happen in the modern 
type of lamp, but with the old lamps it was a constant 
source of annoyance. However, the projectionist will do 
well to dust off the top of his carbon arms, particularly the 
insulation, every day before he starts the run. 



The Projector 

from a little sheet iron affair about six inches wide by 
twelve inches long and twelve inches high, to an impos- 
ing structure of very ample dimensions. It is well that it is so, 
because a roomy, well constructed, well ventilated lamp 
house is essential to high-class work in these days of high 
amperage and a brilliantly lighted screen. 

In the early days when it was the exception to use in ex- 
cess of 25 amperes for projection, and 30 was about the 
limit, very little attention was paid to the housing of the 
lamp. The main object was to provide a holder for the 
condenser and to confine at least a part of the light. With 
the modern projection arc, in some instances using as high 
as 120 amperes, the tremendous heat generated has com- 
pelled close attention to lamp house ventilation and has 
compelled the increase in size before mentioned, while the 
advance in projection optics and the tendency to breakage 
of lenses through the heat of high power arcs has obliged 
manufacturers to give close attention to the condenser 
mount, all of which has resulted in a vastly improved and 
very efficient lamp house. 

LAMP HOUSE VENTILATION.— The ventilation of the 
lamp house is of extreme importance, especially where high 
amperage is used, since unless there be ample air circulation 
the temperature inside it will reach a very high degree, 
which will automatically operate to (a) reduce the capacity 
of the carbons, (b) injure the wires on the interior of the 
lamp house, as well as to some extent the metal of the car- 
bon jaws themselves, (c) set up a tendency to abnormal con- 
denser breakage and (d) through heat radiation make it 
both uncomfortable and unhealthful for the projectionist in 
southern climates, or during warm weather in more north- 
ern latitudes. 

One cause for poor lamp house ventilation is chargeable 



to what can be termed nothing else than pure unadulterated 
carelessness or laziness on the part of the projectionist, 
who allows the vent screens to become clogged either 
almost or quite solid with ash and dirt. 

In the process of volatilization water glass, which forms 
the binder of the core of carbons, produces a gray colored 
ash which is very light in weight. It is this ash which forms 
the white coating found at the top of and on the interior 
walls of the lamp house. It is carried upward by the drait 
and gradually chokes the perforated metal used for vent 
screens in the old style lamp house. Unless this deposit be 
frequently removed and the screens thoroughly cleaned, all 
ventilation will be stopped in a comparatively short time. 
It is the failure to keep these screens free from this accu- 
mulation which we have charged to carelessness or laziness 
on the part of the projectionist. We do not like to use such 
harsh terms, but the fact remains that this condition is 
responsible in altogether too many instances where con- 
denser breakage is complained of. 

of ventilation is illustrated in Fig. 104, in which a three or 
four-inch sheet metal pipe is attached to the top of the 
lamp house, and is either carried to the open air or con- 

nected to the vent flue. 
fly* I Some years ago the 

author secured the 
consent of the leading 
projector manufactur- 
ers to the placing in 
the top of their lamp 
house an opening to 
which such a pipe 
could be attached. The 
installation of a vent 
pipe of this kind 
serves to carry away 
very much of the heat 
of the arc, hence re- 
duces the liability to 
condenser breakage 
and renders the posi- 
tion of the projection- 
ist far more tolerable 
during the hot sum- 


Figure 104. 


mer months. It has the hearty endorsement and ap- 
proval of the projection department of Moving Picture 
World and the author of this book. The author would ear- 
nestly recommend its installation in all projection rooms, on 
the score of health if there were no other reasons; because 
the pipe carries away all fumes of volatilizing carbons 
which are not especially healthful. In installing the pipe, 
however, remember that only a part of its purpose is 
served if you merely run a short piece of pipe up a foot or 
so abqye the lamp house. This ventilates the lamp house 
all right, but it does not remove from the room either the 
heat of the arc nor the gases formed by the volatilizing of 
carbon; also it would not be approved by the authorities in 
some cities. 

Run the pipe out to the open air, into the projection room 
vent flue or into the exhaust pipe if the projection room is 
connected with the house ventilation system. 

It is not necessary that this pipe be capped with a screen 
if this is done, because in any event it would not be less 
than five or six feet long, and by no stretch of even the 
wildest imagination would a spark from an electric arc 
carry such a distance. 

SWING JOINT IN VENT PIPE.— If it is necessary to 
swing the lamp house over to a stereopticon lens it can be 
readily done by providing a combined swing and slip joint 
in the lamp house vent pipe. 

Lack of ample ventilation in the lamp house causes ex- 
hibitors as a whole a large sum in condenser breakage 
every day. While no figures are available we believe this 
item will in all probability amount to as much as $500 a 
day in the United States and Canada alone, meaning that 
that value in condenser lenses probably is destroyed which 
would not be destroyed if lamp houses all had ample ventila- 

PROJECTING CRATER IMAGE.— As has already been 
pointed out, from many viewpoints, it is desirable to pro- 
ject an image of the crater to the wall, ceiling, floor or to 
some other place where it will be constantly in view of the 
projectionists. A crater projector designed to be attached 
to the lamp house door may be purchased from almost any 
dealer in supplies, or one may be made as per directions 
given under "Crater Angle," Page 405. 




a very simple matter to place a small porcelain lamp recep- 
tacle in the bottom of the lamp house, at the right hand, 
rear corner. From one side run a wire to one side of any 
convenient incandescent circuit. From the other side at- 
tach to the other side of the circuit through a spring- 
switch, made as per Fig. 105, attached to the right hand 
lamp house wall in such way that a piece of fibre fastened 
to the lamp house door will shove the switch open, thus 
putting out the light, when the lamp house door is closed. 

By the use of a low C-P lamp the interior of the lamp 
house is thus automatically illuminated when one opens the 
door to re-set the carbons, etc. 

CONDENSER HOLDER.— It is only of late years that 

any particular atten- 
tion has been paid to 
the condenser holder, 
but now all the recog- 
nized professional pro- 
jectors have a more 
or less efficient ar- 
rangement both for 
holding the lenses, and 
for spacing them 
properly with relation 
to their distance from 
each other. A con- 
denser holder which 
has no provision by 
means of which the projectionist may alter the spacing of the 
lenses by means of an adjusting screw located outside the 
lamp house and casing is not a good holder. 

WHY CONDENSERS BREAK.— Condenser lenses break 
because one part of the lens, the edge, is thin, and another 
part, the center, is thick, hence when subjected to heat the 
thin edge increases in temperature very rapidly and expands 
very rapidly as compared to its *hick center. Also when 
the arc is shut off the thin edge contracts very rapidly as 
compared to the thick center. 

We believe it was W. G. Woods, a San Francisco projec- 
tionist, who first recognized this proposition and undertook 
to provide a means for equalizing the contraction and ex- 
pansion as between the thin edge and the thick center of 
the condenser lenses. The idea, which has later been 

Figure 105. 



adopted in one form or another by all manufacturers of pro- 
fessional projectors, was to firmly clamp the thin edge of 
the lens in a metal retainer, the amount and kind of metal 
of which was carefully calculated with a view of its tem- 
perature rising as nearly as possible equally with the tem- 
perature of the thick center of the lens, and since the thin 
edge would be clamped in this metal ring, radiation would 
prevent it from heating faster than the metal holder, and 
the whole lens would thus be brought up to temperature 

Figure 106. 

evenly, and by reversal of the process would lose its tem- 
perature evenly when the arc is shut off. 

The first of these holders, known as the "Elbert Holder," 
is shown in Fig. 106. It was the holder designed for the 
Powers projector by Mr. Woods, who also designed holders 
for the Simplex and the Motiograph. These holders are, so 
far as we know, no longer marketed. 

PREDDY HOLDER.— Walter Preddy of San Francisco 
still makes the Preddy condenser holder illustrated in Fig. 



This holder is excellent for use with old style lamp houses 
which are not equipped with the modern condenser holders 
now put out with all professional projectors. They are 
moderate in price and may be had from any first-class sup- 
ply dealer, or from Walter Preddy, San Francisco, Cali- 
fornia. Their method of mounting is shown at the right in 
Fig. 107. 

requirements of the modern condenser holder are that the 
metal of the holder shall have good, even contact with 
lenses, since unevenness of contact between the metal and 

Figure 107. 

the glass produces uneven radiation, with uneven expansion 
and contraction, and consequent tendency to breakage. It 
must grasp the lens firmly enough to insure good contact 
between the metal and the glass, with consequent evenness 
of radiation, but at the same time not in any way binding 
the lens, because the ratio of expansion of glass and metal 
is different, and if the lens be bound tightly in the holder, 
expansion will exert such a tremendous force that breakage 
will almost inevitably occur. The metal in which the lens 
rests must be so calculated with regard to its amount that 
the purposes of a heat and cold reservoir as hereinbefore 
set forth will be as nearly as possible perfectly served. The 


holder must be so constructed that by means of an adjust- 
ing screw located outside the lamp house the projectionist 
may alter the distance between the two lens holders in or- 
der to accommodate the difference in thickness of lenses of 
different focal length. The holder must be so constructed 
that while it is held locked securely in place, the lenses 
may still be made immediately and easily accessible to the 
projectionist for removal and replacement, always remem- 
bering that removal and replacement in a minimum of time 
may be necessary when the lens to be removed is very 
hot, and a hot lens is a reasonably difficult thing to handle. 

and generally accepted practice is to place the condenser 
holder inside the lamp house, where the lenses are sub- 
jected to a rather high but comparatively even temperature, 
rather than to the uncertain and possibly rather sudden 
changing temperature of the condenser casing located out- 
side the lamp house. 

SHUTTER FOR CONDENSER.— Some modern lamp 
houses are provided with an interior douser, which comes 
down between the arc and the collector lens, rather than in 
front of the converging lens. This is, we believe, good 
practice, since it to a considerable extent protects the col- 
lector lens from sudden changes of temperature when the 
lamp house door is opened while the lenses are hot. 
Such a shutter may be installed by the projectionist him- 
self, and may be so made that it will be lowered by the act 
of opening the lamp house door and raised by the act of 
closing it, so that the lens will always be protected when 
the lamp house door is open. In order to do this a shutter, 
preferably made of quarter-inch asbestos millboard, must 
be installed in grooves, the latter so supported that when 
the shutter rests on the bottom stop it will cover the in- 
terior surface of the condenser. From the top of this shut- 
ter a light chain may be run up and out through a small 
hole drilled in the lamp house roof or wall, and after pass- 
ing over a pulley be attached to an arm riveted to the lamp 
house door in such way that opening the door will drop 
the shutter, while closing the door will pull it up. We 
believe this description is sufficiently clear to serve the pur- 
pose without the aid of an illustration. 

KEEP THE LAMP HOUSE CLEAN.— The careful, com- 
petent projectionist will keep his lamp house scrupulously 
clean. It is not creditable to the projectionist to find dirt, 


dust, pieces of broken carbons, carbon stubs, etcetera, lit- 
tering the floor of the lamp house. It doesn't impress one 
with the idea that the man in charge is a good workman, 
because it is not a workmanlike way of doing things. 

At least once a week the projectionist should, using a 
good hand bellows, blow the carbon ash out of the vent 
screens down into the lamp house, whence it can be re- 
moved. Of course if the lamp house be equipped with a 
vent pipe, as before described, this will not be necessary, 
though the carbon ash should be swept out of the top of the 
lamp house once a week, merely as a matter of cleanliness. 
The removal of this ash is rather an unpleasant job, because 
being very light in weight it is apt to fly all over everything. 
Our own way of doing the thing used to be to take the lamp 
house off once a week, take it outside and clean it thorough- 
ly, but this is hardly practical with large, heavy modern 
lamp houses. By blowing the dust down into the lamp 
house first, however, and by careful work thereafter, very 
little, if any, of it should escape into the room. Lamp 
houses should have an opening at the bottom through 
which dust and dirt can be swept. The removal of dirt from 
the bottom of a lamp house not thus equipped is a tedious 
and a decidedly unpleasant task. 

Many projectionists who are using old type lamp houses 
will find that when the lamp has the desired angle, the 
lower carbon jaw will come into contact with the front 
wall of the lamp house, thus charging the whole projector 
with EMF. Where this occurs the projectionist should 
secure a thin piece of asbestos millboard (sheet asbestos 
will do), and attach it to the front wall of the lamp house 
in such way that it will come between the lower carbon jaw 
and the metal of the lamp house. 

If the lamp house is of the old, unlined, narrow type it is 
an excellent plan to rivet l/8th-inch asbestos millboard, or 
sheet asbestos, to the left hand wall or door opposite the 
binding posts of the lamp, since many annoying grounds 
are caused by a stray strand or strands of the lamp leads 
protruding and making electrical contact with the lamp 
house wall. 

THE LAMP. — The projector arc lamp is a most impor- 
tant feature in the production of good projection light. It is 
in fact impossible to secure consistently even screen illumi- 
nation with a poor, badly worn, loose, "wabbly" or dry lamp. 
Theatre managers who oblige their projectionists to work 


with an old style arc lamp are doing a very foolish thing, 
and one which cannot but result in inefficient work on the 

To be in accord with modern practice a projector lamp 
must have an adjustment for feeding the carbons; an ad- 
justment by means of which one or the other of the carbon 
jaw may be moved sidewise ; an adjustment by means of 
which the whole lamp may be moved forward and back; an 
adjustment by means of which the whole lamp may be 
raised vertically up or down; an adjustment by means of 
which one or the other, preferably the lower, carbon jaw 
may be moved forward or back with relation to the other 
jaw; an adjustment by means of which the whole lamp may 
be moved sidewise; and an adjustment for altering the angle 
of the lamp as a whole. All these adjustments, except the 
last named, must be available to the projectionist from the 
exterior of the lamp house. In addition to this many pro- 
jectionists demand that there be means provided for the 
tilting of the carbon jaws to accommodate the "jack-knife" 
carbon set, but tke author believes that just as good, if not 
better results would be had if this last adjustment were 
entirely omitted. 

INSULATION. — The carbon jaws must of course be in- 
sulated from the carbon arm, the jaws being the only part 
of the lamp electrically charged. This insulation is com- 
posed of sheets of mica, and there is of course an insulating 
"barrel" of mica around the screws or bolts which clamp 
the jaw to the lamp arm. Projectionists should be very 
cautious about loosening the joint between carbon arm and 
jaw, because if the insulation around the bolts or screws be 
injured or destroyed considerable trouble is likely to be 
experienced in replacing it. 

CARBON DUST GROUNDS.— The projectionist should 
be careful to keep the lamp free from carbon dust, partic- 
ularly around the insulation, since it is quite possible to 
form a current-carrying ground through carbon dust set- 
tling on the top of the surface of the carbon jaws and arms. 
Before projector manufacturers adopted the practice of 
raising the edge of the mica insulation above the surface of 
the metal at the top of the carbon arm and jaw, carbon 
dust formed a prolific source of grounds in the lamp. This 
can no longer occur unless the projectionist is careless 
enough to allow a very considerable amount of dust to 


CARBON CLAMPS.— It is of very great importance that 
the carbon jaw make as nearly as possible perfect electri- 
cal contact with the carbon, since otherwise heat will be 
generated; also there will be more or less arcing between 
the carbon and the metal, which will tend to gradually 
roughen the jaw by forming pit-holes in its surface, thus 
setting up a very bad condition indeed. Any heat caused 
jDy poor contact between the metal and the carbon increases 
tendency to penciling of the carbon, because it adds to the 
heat of the arc the heat formed by the poor contact. A 
good plan for keeping the contact in good condition is to 
wrap a round piece of wood about the diameter of the car- 
bons being used with No. 00 sand paper, or very fine emery 
paper or cloth, and every day before starting the run clamp 
this lightly into the carbon jaws and give it a few twists. 
This will remove any scale or other thing adhering to the 
carbon jaws which might tend to increase the resistance 
between the metal and the carbon. The cleaner above de- 
scribed is easily made by cutting sheets of sand or emery 
paper into strips an inch or so wide, and winding them 
spirally around the cleaning rod tightly, fastening the up- 
per end with a thumb tack such as draughtsmen use. 

LAMP LUBRICATION is an exceedingly important thing. 
Projectionists should make it their practice to lubricate 
their lamp thoroughly at stated periods. Twice a year all 
screws should be removed from the lamp, dipped in kero- 
sene and then into a box of powdered graphite, the oil 
merely being intended as a binder to hold the graphite to 
the screw until it can be replaced. Moving parts should be 
lubricated by rubbing them with a cloth wet with kerosene 
and then with graphite. If kerosene is not available lubri- 
cating oil may be used as a binder for the graphite, but it 
must be remembered that the oil is not intended as a lubri- 
cant, but merely as a medium to hold the graphite to the 
metal until it is thoroughly coated. The graphite itself is 
the lubricant, graphite being in itself a high-grade lubri- 
cant and one which is impervious to the action of heat at 
ordinary temperature. In reassembling the lamp, remember 
that the greater the amount of graphite adhering to screws 
and moving parts the better. If you have never done this 
you will be astonished at what a difference it will make in 
the handling of the lamp. 

Make it your invariable practice to remove the carbon 
clamp screws every day if the run be a twelve-hour one, or 


in any event frequently, and lubricate them with graphite 
as above set forth. Do this and you will not need to twist 
up the screws with a plier. In fact, if you have been using 
unlubricated carbon clamp screws you will be very likely 
to crush the first few carbons you put in. 

ASBESTOS WIRE LAMP LEADS.— The asbestos wire 
lamp leads are a thing concerning which the projectionist 
must use care and intelligence, else he will have heavy loss 
by reason of their high resistance. 

ARC CONTROLLERS.— There are now on the market 
several arc controllers designed to automatically feed the 
carbons. The use of these controllers cannot be too highly 
recommended, since they maintain an absolutely steady arc 
length, hence an absolutely steady screen illumination, al- 
ways of course provided they are proprely adjusted and 
handled by the projectionist. 

The hand-fed lamp never has and never will give as 
steady screen illumination as that provided by a good arc 
controller. The principal upon which most arc controllers 
operate is as follows : 

Every change of arc length means a change in arc voltage. 
IVhen the arc is burning with a given carbon separation 
there is a certain difference of potential between the car- 
bons called "arc voltage." As the carbons burn away the 
distance between their tips is of course increased, which 
means that the resistance of the arc is increased, hence a 
higher voltage is necessary to force the current across the 
gap. In other words added distance between the carbon 
tips raises the arc voltage automatically. Automatic arc 
controllers make use of this fact and depend upon it for 
their action. When the carbon burns away sufficiently 
to alter the arc voltage by a very small amount, a mechan- 
ism is engaged which feeds the carbons together until the 
normal arc voltage is reestablished, whereupon the mech- 
anism automatically ceases to function until the arc voltage 
again rises sufficiently to re-energize it. 

A good arc controller operates on such slight change in 
carbon separation distance that the feeding of the carbons 
is well nigh imperceptible, hence a practically uniform dis- 
tance of carbon separation is automatically maintained. In 
Volume II will be found more detailed descriptions of arc- 
controllers now on the market together with characteristics 
as set forth by their manufacturers. 




Jean A. LeRoy's "Marvellous Cinematographe" 

Started in September, 1892, finished February, 1893, to run unperforated 
films. Remodeled January, 1894, and finished February 3, 1894, to run 
perforated Kinetoscope film. 

First public showing February 5, 1894, and in use until July 6, 1897, 
using perforated films same as in use today. 



LIGHT is the foundation of projection, and light for projec- 
tion purposes depends to a very great extent for its steadi- 
ness, its brilliancy and tone, upon the arc lamp electrodes 
(carbons), since practically the entire available illumination from 
the ordinary arc is produced by the incandescence of the 
floor of the "crater" formed by the current action on the tip of 
one (the positive) of the carbons if D-C is used, because one 
carbon is then constantly positive, or of both carbons if A-C is 
used at the arc, since then both carbons are alternately positive 
and negative. However, with the use of the white flame 
carbons on A-C the highly luminescent arc flame is used as 
the light source, rather than the crater. The high intensity 
arc, on the other hand, operates on an entirely different 
principle ; the intensely brilliant illumination from this type 
of arc is due to a highly incandescent gas ball held in the 
deep, pocket shaped crater of the positive carbon. 

HOW THEY ARE MADE.— The procedure of American 
carbon manufacturers in the making of projection carbons is 
as follows : The basic ingredient of projector carbons is lamp- 
black, the purest form of commercial carbon known. The 
ordinary lampblack used in the manufacture of other types 
of lighting carbons contains far too much ash to be satisfac- 
tory, therefore a specially selected black is employed. Even 
this material contains considerable volatile matter, which is 
driven off by calcination at a high temperature. This cal- 
cined material is known as "carbon flour," and is so pure that 
it is less than one-twentieth of 1 per cent (.0005) ash, and con- 
tains little or no volatile matter. A high grade binder is then 
added to this flour, after which it is machine-mixed into a stiff 
mass, in a fashion very similar to that employed in kneading 
bread dough. This mass is then fed into the cylinders of hy- 
draulic presses, which force it through suitable dies under 
verjr heavy pressure. As it comes from the presses the carbon 
is received upon grooved boards, made for the purpose. It 
is now in the form of rods. Carbons which are to be cored 
are forced with a central hole throughout their length, formed 
by having a steel pin fixed in the center of the hole in the die. 


At the end of the process just described the carbons are ready 
for baking. The form of the binder contained in the green 
carbon must be changed by driving off the volatile matter 
therein contained and depositing the residue throughout the 
electrode in the form of pure carbon. Inasmuch as the 
quality of the finished carbon depends to a large extent upon 
the method and temperature of the baking, this is one of the 
most important operations in its manufacture. The green 
carbons are first packed in special cylinders, to keep them 
from becoming crooked and to protect them from injury. 
They are then placed in gas fired furnaces, specially designed 
to secure uniform heating, from which air is excluded during 
the process of baking. The total operation of packing, baking, 
cooling and unpacking consumes from three weeks to a 

After removal from the furnace the carbons are cut to 
proper length and sorted for straightness. Owing to variation 
in shrinkage during the baking process, some deviation from 
perfect straightness must be expected. The solid and hollow 
carbons are now separated. The former are taken directly to 
the pointing machine, after which they are ready for shipment ; 
the latter go to the coring department. Here the central hole 
in the carbon is filled with the core material, which is a non- 
flaming, arc-supporting substance. The core material is mixed 
into a paste with water glass, a soluble akaline silicate which 
becomes solid when dried. It is then forced into the hole, 
after which the carbons are rebaked for a short time at a 
comparatively low temperature in order to solidify the cores, 
which operation completes the process of manufacture. 

PURPOSE OF THE CORE.— In order to understand the 
reason why it is necessary to place a core in carbons used for 
the positive electrode of a projection arc lamp, we must first 
understand a few of the whys and wherefores of the arc itself. 

One may close the projector table switch, thus charging the 
carbons with EMF, and bring the carbon tips within l/64th 
of an inch of each other without results of any kind. So long 
as there is an air gap between the carbons, no matter how 
small it may be, neither 110 nor 220 volt current will jump the 
space. Yet when the carbons tips are brought into actual 
physical contact with each other, and current flow is thus 
started, the carbons may be separated and the current will 
continue to flow, even after the distance of separation has 
increased to as much as three-eighths or half an inch, and 
under some circumstances even a very much greater distance. 


The novice usually is puzzled to account for this phenomenon, 
the explanation of which is simple. 

Current of the voltage used for projection will not jump an 
air gap of any width at all, but when the carbons are brought 
into contact and separated, at the instant of separation the 
action of the current heats the carbon particles to the point of 
volatilization, and in the process of volatilization a gas is 
formed which to a considerable extent is a conductor of elec- 
tricity. From the instant the carbons are separated until they 
are separated so far that the resistance of the gas stream is 
too high to allow of the. EMF forcing the current across the 
gap, this gas exists between the carbon tips, and furnishes a 
conductor for the current. 

With this in mind let us examine into the reason for the 
placing of the core in the positive carbon. Electric current 
always seeks the path of least resistance, and where several 
paths are available a very slight change in resistance may 
alter the path of the current. Were we to attempt to use a 
solid positive carbon a crater would be formed the same as 
with a cored carbon, but since it would be utterly impossible 
to secure a carbon mass which would always have the path 
of least resistance in one spot, the center of the crater, the 
main flow of the current would, seeking the path of least re- 
sistance, move around over the face of the crater wherever 
at any given instant of time the least resistance was offered 
to its passage, which of course, since the light for projection 
must be absolutely steady, would not do at all. 

At the incandescent crater floor the core supplies a far 
greater volume of arc supporting gas than does the material 
composing the surrounding shell, or wall of the carbon, hence 
the current, seeking the path of least resistance, is led by the 
relatively heavier gas volume to the core of the carbon, and 
by the above described condition, is kept there, which has the 
practical effect of maintaining the center of the crater at the 
core, with resultant steadiness of illumination at the plane of 
the collector lens. 

For extremely low direct currents a cored negative is some- 
times necessary with the cored positive while 85 amperes and 
above it is always necessary to use the cored negative. At 
nominal currents a solid negative carbon will generally give 
the best results. We are speaking now of the high quality 
National Orotip Negative Projector carbons and not the ordi- 
nary large diameter solid and cored negative projector carbons. 

SOLID VERSUS CORED.— The old practise was to use a 


large diameter regular cored or solid projector carbon as the 
negative ; the carbon being only slightly smaller in diameter 
than the positive carbon, a Y$ inch positive and a % inch cored 
or solid negative. A great many projectionists still continue 
to use such combinations but they are not only sacrificing 
light but obtaining poor screen illumination as well. With 
the ordinary large diameter cored or solid negative carbon a 
large blunt point is obtained and the arc instead of remaining 
on one spot wanders or travels around this large blunt point 
with the result that the light on the screen is unsteady. In 
addition, the large blunt point being in the path of the light 
rays obscures part of the light emitted from the crater. In 
order to eliminate these undesirable features several years ago 
the National Carbon Co., Inc., invented and developed the 
"Silvertip" Negative Projector Carbon, the first small diameter, 
metal coated negative carbon on the market. This carbon 
greatly improved the character and steadiness of the light 
from the D-C arc. However, the Research and Development 
Laboratories of the National Carbon Company are continually 
investigating means for placing their product on an even higher 
plane of quality, so recently they discontinued the manufacture 
pf "Silvertip" and are now producing a new improved nega- 
tive carbon called the "Orotip," a carbon which they claim 
has longer life, greater steadiness and less condenser pitting 
than any other metal coated negative carbon on the market. 
The small diameter of the "Orotip" keeps the arc steady and 
it does not obstruct the light rays to any appreciable extent. 
While it is true that the initial cost of the old style, large 
diameter negative carbons is less than the new "Orotip" metal 
coated negative, yet the motion picture theater has only the 
projected picture to sell to the public, and if a decided im- 
provement in the quality of the light projected on the screen 
costs only a few cents a day more, the slight additional cost 
certainly is a good investment. Therefore, we can recommend 
this negative carbon to the projectionist as being decidedly 
better than either the plain cored or solid carbon as a negative. 

CARBON SIZE. — The diameter of carbons is a matter of 
very great importance to the projectionist who desires to do 
his. work efficiently, and to get the greatest possible amount 
of projection light per watt of current consumed. If the car- 
bon be too small there will be "pencilling" or "needling," or 
"spindling," which is the burning of the carbon tip to a more 
or less long, slim point. The result of this phenomenon is to 
produce rapid consumption of the carbon, reduced crater area 


if the splindling occurs in the positive carbon, or endangering 
satisfactory operation if it occurs in the negative. With alter- 
nating current the effect would be much the same as that 
encountered in the D-C positive carbon. 

If, on the other hand, the carbon be too large, then it seems 
certain that the comparatively large mass of relatively cool 
carbon lying close to the floor of the crater will, at least to 
some extent, operate to lower the average crater temperature, 
which must inevitably reduce its candle power per unit of area. 
It therefore follows that carbons either too small or too large 
for the amperage used are not efficient. 

TION EFFICENCY.— In the "Electrician," Volume 32, pages 
117, 145, 169, year 1893, Blondell says that although the maxi- 
mum brilliancy of the crater is independent of the current 
flowing in the arc, yet the average brilliancy of the incan- 
descent portions of the crater increases, both with intensity 
and density of current, until the crater is well saturated. "If," 
he continues, "the volume of the current be suddenly altered, 
the intrinsic brilliancy undergoes a temporary but very ap- 
preciable variation, which may reach 10 per cent., but which 
diminishes gradually until the dimensions of the crater are so 
altered as to restore the surface to the value it ought to have 
for the new current." 

Blondell says that the heating of the crater only takes place 
at the surface, and that the temperature of volatilization is only 
reached by a very thin, superficial layer, all of which seems to 
make plausible our argument that a comparatively large body 
of relatively cool carbon near the floor of the crater would 
operate to decrease crater brilliancy. 

Mr. W. R. Mott and Mr. W. C. Kunzmann of the National 
Carbon Co., Inc., presented a paper entitled "Efficiency in 
Carbon Arc Projection," before a recent meeting of the Society 
of Motion Picture Engineers which very clearly explains the 
relationship between intrinsic brilliancy of the carbon arc 
crater and the factors effecting the efficiency of the carbon 
arc when it is used as a source of illumination for projection 
purposes. This paper is published in the Transactions of the 
Society of Motion Picture Engineers, Volume 16, pages 143 to 
154 inclusive. We are indebted to the National Carbon Co., 
Inc., for having made available the extracts from this paper 
which appear in the following paragraphs. 

"The efficiency of the light production in a projection arc 
is determined by two main factors ; first, the intrinsic bright- 


ness per unit of surface of the crater; and second, the effec- 
tive area of the crater. By effective area is meant, of course, 
the horizontal projection of the actual crater area on a plane 
perpendicular to the optical axis ; and consequently the angle 
which the carbon carrying the crater makes with the optical 
axis becomes an important consideration. It is at once ap- 
parent that right angle operation of any carbon is a material 
factor in its light producing efficiency. 

The carbon arc has the greatest brightness per unit area 
of any known light source, hence becomes ideal for picture 
projection. The positive crater of the ordinary carbon arc is 
about three times as bright as molten tungsten, which in turn 
is the upper limit of the brightness of the incandescent lamp. 
This factor augments the supremacy of the arc where optimum 
illumination or long projection distances are desired. 

The brightness of the positive crater of a carbon arc is fixed 
by the sublimation or vaporization temperature of carbon, 
and is consequently a definite and constant value. For just as 
water in an open pan cannot be raised to a higher temperature 
than its boiling point, so carbon cannot be heated higher than 
its vaporization or sublimation point at atmospheric pressure. 
At the surface of the positive crater, energy is supplied 
chiefly as the product of current and anode voltage, the anode 
voltage naturally varying from 35 to 40 volts with usual carbon 
arc. This energy is dissipated in the following three channels: 
(1) radiated energy, including infra red, visible light, and ultra- 
violet, (2) loss of heat through conduction by the carbon, and 
(3) the latent heat of vaporization of the carbon itself. In a 
high grade lampblack carbon the heat conduction loss is made 
very small, with consequence that the radiant energy may 
equal half of the total energy supplied at the positive. This 
radiant energy is emitted at the enormous rate of 135 watts 
per square millimeter of the positive crater surface. If the 
electrode was made of graphite there would be an inherently 
high conductivity, so that the positive crater would become 
very small (about one-third of the area of the crater of a good 
lampblack carbon). 

Carbon materials for projection use are therefore selected 
largely because of their low heat conductivity and consequent 
ability to permit high concentrations of heat energy. With the 
usual carbon arc the evaporation of carbon is a minor factor, 
though it is possible by using carbons of a small diameter at 
excessive current densities, to make the evaporation factor a 
major one. 


One of the important assets of the carbon arc is its flexibility, 
permitting wide adjustment of screen light by changes in the 
current. This is especially important with dark or colored 
films where the same arc lamp may be satisfactorily used over 
a wide range of conditions. The relations of carbon diameter 
and amperage to candle power we will now develop more 
fully, as of special interest to the projectionist. 

While the crater area (and hence the light) increases with 
the current, it also depends to a surprisingly high degree, on 
the carbon diameter. The relation of carbon diameter to crater 
area,* with a constant current value, is shown in Fig. 108, in which 
it will be noticed that the positive crater diameter (and hence 
the area) falls off with all the smaller carbons, and passes 
through a maximum with the large solid carbons. With the 
small carbons, a high current density results from the con- 
centration of current on the electrode tip, the crater area being 
naturally limited by the size of the carbon. (With very small 
carbons, the diameter of the crater over its actual surface is 
greater than the diameter of the carbon. This peculiar result 
is explained by the crater overlapping the sides of the carbon 
like a cap, making the true crater greater than the cross- 
section area of the carbon, hence the true crater diameter 
greater than the diameter of the carbon.) With the larger 
carbons, the crater area is reduced from its maximum because 
of the heat conductivity of the large mass of carbon behind 
the crater. There are also given results under like conditions 
for a neutral cored carbon and a solid flame carbon, both of 
which give smaller craters than the solid carbons. 

For the sake of increased arc steadiness, it is customary to 
use cored positive carbons. For the negative we recommend 
a small carbon plated with a metal of an oxidation resisting 
character to give high current carrying capacity. The object 
of the small negative is the decrease in the interception of light 
from the positive ; a second effect is to hold the positive crater 
in a constant position which permits a slightly lower arc volt- 
age to be employed. This combination is an American inven- 
tion of considerable merit. For very high currents, metal 
plated cored negatives may be substituted for the solid. 

The sizes of positive projector carbons commonly recom- 
mended for different currents are shown in table 24. 

For equal amperages, direct current gives more than twice 
the light produced by alternating current, which is a valid 
reason for changing to direct current. Moreover, theatre 
managers and projectionists have come to realize that large, 




of Crater <S/ze to Carbon S 





All Tests at 2S Amps. D. C. 





• Core J 


iSol/J rlanto Cor 






Y i 

V J / 


i 2 

i " 

Carbon Viametfir /> A////*mtt«rj 
Fig. 108. 

bright pictures, in a theatre which is itself well lighted, secure 
the most and best paying patronage. The expense of the 
brilliant screen lighting is such a nominal percentage of the 
box office receipts that it is indeed false economy to try to 
operate with anything except the best light sources. A high 
amperage carbon arc results in the best and the most light, 
together with a white quality thereof, which means the most 
satisfactory picture from the viewpoint of audiences. 

To the projection engineer and projectionist it is important 
to know how the light varies with the current, because a 
slight increase in current may mean a considerable gain of 
candle power. This will now be analyzed as regards (a) the 
light produced at the arc, (b) the transmission of the optical 
system, and (c) the light delivered at the screen. 

The total light produced depends, as we have said before, 
upon the crater area. By using larger carbons of the proper 
size for the larger currents, it is found that crater area in- 
creases more rapidly than current. In fact it may be shown 
that when the current is doubled the crater area is increased 




C 6. 

Projection Ejigineeriv\g — <Seorch](gts 
Area of Crater — Core J Positives' 

See Bool Rey -Pa£e 25 Electric SearcM^Jit Projectors 
Curve I- Crater Area to Current- Large Positives 
Curve H.- Crater Are* to Current- Si 






^>,» ,v 





o~ io id To +o jo to 70 eo 90 Too no no 73? "mo Tto tee 170 mo i$o 100 
Cur-rent /» Amperes 

Fig. 109 

approximately 2.5 times. The effect of this becomes impressive 
when wide ranges of current are considered. 

The relation of current to crater area and to candle power 
is shown in Figs. 109 and 110. 

20 JO -40 SO 60- 70 60 90 100 110 120 /JO 1+0 1 90 

Current in Amperes 

Fig. 110 



A series of tests was made of the positive crater area of 
direct current trims with the following results : 

TABLE No. 19 
Positive Crater Area at Different Currents 

Size of Carbon 


er Area in 




square inches 



















The candle power of the arc follows a curve similar to that 
of the relation of crater area to current. This is illustrated 
in Fig. 110. When the current is doubled, the candle power 
increases 2.55 times. 

Relation of Screen 
I At first the screen candle 

Candle / 
power a 

' light s 
d the | 

w«r to 

Area of Light Source 


_ Z, 


of the 
At a cer 

light sou 
tarn <Jia 
rs con 


rreler of 
not col/e 

ource. (. 


? limit >s 


at which 




/Urea of Lighl Source 

Fig. Ill 

In Fig. Ill the behavior of the optical system with increasing 
area of light source is shown. With small areas of light 
source, the light delivered increases proportionally to the area 
until the edges of the light source are beyond the optical 
reach of the light collecting system. Light generated by a 
crater of larger size than can be accommodated by the optica? 
system is wasted. 



The combination of the curve of efficiency of the optical 
system and the curve of the efficiency of the original light 
source gives a compound curve as shown in Fig. 112. At first 
the screen light increases very rapidly with the increased 
current and then, because of conditions in the optical system, 
the rate of increase is diminished. It is important to note 
that a change of optical system will change the resultant effi- 
ciency and a suitable lens combination is consequently a valu- 
able asset. 

* ie 3.0 ao +o so eo to 00 90 100 Tfo fto T30 mo jso 

5— To — eo to bo 90 iho no /j 
Direct Current Amperag* 

Fig. 112 

The results shown in Fig. 112 are for a particular optical 
system. It is not claimed that the best carbon setting was 
obtained, for skilled projectionists can often show surprising 
gains in candle power over laboratory operators. The curve 
illustrates, however, the marked gain in candle power for 
small increases at the lower currents and the correspondingly 
lesser gains at currents around one hundred amperes. This 
relation for different amperage ranges expressed in per cent, 
increase in screen candle power is shown in the following 
Table 20. 


TABLE No. 20 

Current Increase Per Cent. 

(Amperes) Gain in Screen 

From To Candlepower 

30 50 160 

50 70 46 

70 90 18 

90 110 15 

110 130 12.5 

TION EFFICIENCY.— Flame materials are used in the White 
Flame A-C projector carbon and in the White Flame High 
Intensity D-C carbon, both of which are covered by American 
and foreign patents. The first to be used was the White 
Flame A-C projector, which has the following distinct advan- 
tages as compared with the neutral cored projector carbon: 


Better illumination. 


Wide range of opera- 


Whiter light. 





Better crater formation. 


Better power factor. 


Decreased arc voltage. 


Silent arc. 


Clearer pictures. 

The White Flame A-C projector carbons give better illu- 
mination, especially at high currents. The color of the light 
is also markedly improved by the use of these carbons where 
an alternating arc with neutral cored carbons gives a decidedly 
yellow light compared with the D-C carbon arc, the white flame 
chemicals correct this almost perfectly by supplying the arc 
stream with the blue needed with the yellow craters to give a 
good daylight white. This white light is well known to give 
better viewing power. 

The use of the white flame materials gives a much steadier 
light. The arc with the neutral cored carbons has a tendency 
to "chase" occasionally, which is the result of the arc leaving 
the crater and moving down and around the sides of the carbon. 
The high arc supporting power of the white flame materials 
holds the arc in its proper position with no tendency to chas- 
ing or unsteadiness. 

From the projectionists' standpoint, the greatest practical 
advantage is the reduction of noise. An A-C arc with neutral 
carbons at 65 amperes makes a very loud, roaring noise, which 
increases at higher amperages. The White Flame A-C re- 
duces this noise to a very slight hum, so that the motion pro- 
jector ma 1 -es more noise than the arc itself. A series of ex- 
tensive experiments has shown that rare earth compounds are 


the best materials for reducing noise of a high amperage arc, 
though there is a marked difference in the noise suppressing 
power of the several elements of this group. The use of these 
materials has been largely covered by patents. 

The white flame projector can be used successfully up to 
100 amperes, whereas the neutral cored carbons can operate 
satisfactorily only to 65 amperes. This illustrates the wider 
range of operation that is feasible with the White Flame A-C 

The crater formation at 65 to 100 amperes is far better with 
the white flame projector carbons. 

The pictures projected with the White Flame A-C projector 
carbons are much better than with the neutral carbons. They 
show better perspective, better penetration and more detail." 

The foregoing paragraphs explain very clearly how the mo- 
tion picture projectionist can obtain the most light and the best 
results from the equipment placed at his disposal and thus make 
sure that the patrons of his theatre are enjoying a well lighted 
picture. The excellent photographs, Fig. 113 and Fig. 114 illus- 
trate the points brought out by Messrs. Mott and Kunzmann 
with regard to "penciling" or "spindling." 

Fig. 113 is an actual photograph of the crators of three sizes 
of cored positive carbons burned at various currents. The 
Y§ inch positives were burned at 25, 35, 45, and 55 amperes D-C ; 
the % inch cored positive carbons were burned at 35, 45, 55, 65 
and 75 amperes D-C; while the % inch cored positives were 
burned at 45, 55, 65, 75, 85 and 95 amperes D-C. In each case 
the positive carbon was trimmed with the negative recom- 
mended for the conditions being observed. Fig. 114 is an actual 
photograph which illustrated the normal penciling of a cored 
positive carbon on D-C in one case and very excessive pen- 
ciling in the other. The picture of the normal condition was 
obtained from a Y% inch cored positive carbon burned at the 
maximum recommended current for that carbon, while the 
picture of the excessive penciling was obtained from a similar 
carbon that had been very heavily overloaded. 

From the discussion in the paragraphs by Messrs. Mott and 
Kunzmann and by an examination of these photographs, it 
can be concluded that the best results will be obtained from 
a set of carbons by burning them at the greatest current they 
will carry without increased penciling or producing an un- 
steady arc. 

INSPECTION. — The projectionist should carefully examine 
each bundle of carbons before using. Cracks running length- 
wise of the carbons are in a way characteristic of the product, 



% Inch 

Cored Positives 








^4 Inch Cored Positives 
65 55 45 



Yz Inch Cored Positives 
45 35 


Numerals represent amperes D-C 
Fig. 113 


Fig. 114 

and do no harm. They are caused by slight error in the con* 
sistency of the paste from which they are formed. Chip cracks 
running around the circumference, however, condemn the 
carbons, since there would be a tendency to break off at this 
point, though hair cracks are often found running around 
the circumference of good carbons. They are due to the same 
cause as the longitudinal cracks, and are of no consequence. 

EXAMINE CORES.— The ends of cored carbons should be 
carefully examined, and if too many of them show indication 
of imperfection in the core they should be rejected. It is 
absolutely essential to high class work on the screen that all 
cores be thoroughly continuous, also that the core adhere to 
the wall of the carbon sufficiently well to prevent short sec- 
tions of it dropping out as the carbon is consumed. Carbon 
with a section of its core lacking is one of the most annoying 
things the projectionist is called upon to deal with. Absence 
of the core not only produces unsteadiness in the light while 
that section of the carbon is burned away, but also alters the 
tone of the light, and if the center of the crater is in focus at 
or near the film plane, the absence of the core will produce a 
dark spot at the center of the screen. 

We are assured by the manufacturer that all carbons pro- 
duced by the National Carbon Company are very carefully 
inspected for these various faults and all carbons containing 
any of them are rejected and do not leave the factory. 

HARD SPOTS in the carbon due to faults in manufacture 
were of very common occurrence in the early days of projec- 
tion, and were a source of great annoyance to the projectionist. 
Such spots are believed to be caused by the lack of thorough 
mixing of the carbon dough in the early stage of manufacture. 
They are, however, now so seldom encountered that they may 
be said to have been to all intents and purposes eliminated. 



CARE OF CARBONS.— It is essential to good results on 
the screen that the projection carbons be thoroughly dry. 

Fig. 115 

Carbons should, therefore, not only be stored in a dry place, 
but thorough dryness should be further insured by mild heat- 
ing for a day or two before using. This latter may be ac- 
complished by attaching a pair of hooks to the lamp house 
as indicated in Fig. 115, but it is recommended that projector 
manufacturers provide some sort of carbon receptacle either 
in or on the top of the lamp house, capable of containing from 
six to a dozen carbons, both negative and positive. 

BURNING CARBON STUBS.— Modern projection lamps 
accommodate 6 inch lower and 12 inch upper carbons, but it is 
not always practical, particularly where 2,000 foot reels are 
used, to burn carbon stubs very short in the regular way. 

There are on the market any number of "carbon economizers" 
which are nothing more nor less than a metal shank at the 
top of which is a receptacle in which the carbon stub is 


clamped by means of a suitable mechanism. This arrange- 
ment clamps in the regular lamp carbon jaw and permits of 
the carbon stubs being burned down until only an inch or 
two remains. They may be had of any supply dealer. 

EQUIVALENTS. — The equivalents of millimeters in frac 
tions of an inch will be found on page 395. 

Note : — On the opposite page 
is a very genuine curio which 
was deemed worthy of space 
even in this crowded book. It 
is a page from the Edison 
catalogue, issued about 1896, 
showing the Edison spool-bank 
projector, and giving details 
concerning same. 


The Edison Projecting Kinetoscope. 

New Results. New Price, Life Pictures, Life Size. 

Cut Showing Spool Bauk 
. . . CONSISTING OF . . . 
Projecting^ netoscope with Spool Bank, 

Condensing Lens, Objective Lens, 

Electric Lamp, Lamp House and Resistance, 

. Price, Complete, $75.00 

A new projecting machine that accomplishes the results of the 
higher-priced machines, with equal accuracy and more brilliant effects. 
Compact — weighing only seventy-five pounds complete. Portable — 
packed in one case and can be shipped as baggage. Operated by 
hand power, and either 110-volt direct, or 52 or 10-4-volt alternating 
current used for lamp, about 25 amperes giving best results. Storage 
battery cannot be used but other light than the electric can be applied 
if necessary. The pictures projected are life size, and the size of the 
projection on a, screen, at a distance of fifty feet, is 11x13 feet. The 
outline of the pictures is sharp and clear, and Mr. Edison's new 
apparatus has almost entirely overcome the vibration, which heretofore 
has been the principal defect in projecting machines. Machines are sold 
outright, without territorial restrictions of any kind. . 

If electric current is not available we recommend calcium (oxy hydro- 
gen) light. We can furnish a calcium light burner (retort) for $15.00 
extra, and a complete outfit for generating gases, &c , for $125. 


The Light Source 

WHERE the electric arc is the source of projection light, 
practically all illumination available for use comes from 
what is known as the "crater," which is a more or less 
saucer-shaped depression formed on the tip of the positive carbon 
by the action of the current. When direct current (D-C) is used 
at the arc there is only one crater formed, because a crater 
forms on the positive carbon only, and with D-C one carbon 
is always positive and the other always negative. When 
alternating (A-C) is used at the arc, however, a different con- 
dition is set up, because a crater always forms on the tip of 
the positive carbon, and with A-C